996 squealing from engine bay. Possible AOS failure. Ideas? *video attached*

Hello all! A few months ago my 2001 996 C4 made a high pitched squealing noise that started while driving in town. When I arrived home, I shut things down and thought I'd drop by my indie to check on it the next day, but it fired up with no noise and has been running quiet as a kitten until this morning. I left for work and about 10 minutes into my drive, the squeal began so I pulled into a parking lot and took a quick video to capture the sound, which I've posted on YouTube at http://youtu.be/zJMefjFx_y4 . Things I've read here suggest it may be a failing AOS, but I have no oil pressure issues and no smoke eminating from the exhaust - symptoms that seem to accompany that particular issue. Any thoughts would be greatly appreciated. Thanks so much! -Steve

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If not an AOS and the noise may not be due to a failing AOS, then another possible cause is a dry or dragging serpentine belt idler roller/tensioner roller bearing, or a going bad water pump (or other accessory) or a tensioner that has lost some of its tension. One trick is if you know the noise is present at a particular time, is to remove the belt -- note its routing and direction of rotation carefully! -- and then start the engine and let it run only long enough to confirm the noise is still present or not present. Obviously you can't remove the belt from a hot engine and then drive the car down the road for 10 minutes until the noise appears. Nore can you remove the belt from a cold engine and start the engine and let the engine idle 10 minutes until the noise appears. Anyhow, if you can do the above test without risking the engine, and if the noise is still present with the belt removed, then AOS becomes suspect #1. If the noise is not present something related to the serpentine belt/accessory drive is suspect. Sincerely, Macster.

Linky no worky. To rule out the AOS, with the car running, remove the oil cap and put your hand over the opening. You should feel just the slightest vacuum, almost imperceptible. If you have major vacuum, then likely its the AOS.

Change the belt. In my old engine I would get the squak racket out of the belt at start up like it was starter noise. Just the belt being stiff/glazed and not taking to quick increases in rpms like going from 0-1,000 rpm in a split second.After replacing mine I never got another squak out of it. I could of sworn it was a bearing but I guess the bell shape of the pulleys kind of mask the real culprit and amplify the noise. Oh yah, the belt is a quick cheap change you probably are due to do anyway so start there.

Could be a pulley, could be the AOS. How is the coolant level. +1 to run the car very very briefly without the belt on.

I posted the video on YouTube at http://youtu.be/zJMefjFx_y4 - hopefully that will work for everyone in listening to the noise. I will see about testing whether or not it is a pulley. My suspicion is perhaps not because when I shut it down there is a sound like a whistling as that squeal dies down, but I'm no expert so that may just be the sound of the pulley as rotation slows.

Cooling fan?

Dude, that sounds like a running power steering pump with no fluid

Wow, after listening, sounds much more serious. Yes could be something like p/s pump w/o fluid. Should be easy enough to pinpoint if you can duplicate the problem, just remove the airbox and either remove the belt and run it briefly or with something that noisy you might be able to just hear it and see it easily.

I've made my way to my next obligation for the day. I just saw the replies on my phone coming into the building so when I'm done here I'll check on the power steering pump. I should just be able to look and see if the reservoir is dry, right? If it adds anything to the analysis, the sound didn't start fora few minutes into the drive, but once underway it was constant except when I was on throttle. No matter the engine speed, when my foot was off the pedal or just "maintaining" the sound kicked in, but putting the right foot down silenced things. Too bad I can't just accelerate indefinitely!

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+1, just check the p/s fluid level with the dipstick. Remove the airbox and take off the belt after the engine is hot or whenever the sound occurs. It is hard to tell where the noise is coming from in the video and whether or not the camera is distorting the noise. Is does sound like crap... the noise just reminded me when I started the car without p/s fluid in the reservoir. How's the weather out there? I just moved from Bellevue last year, I spent 8 years in Nebraska. Hopefully you can pinpoint the sound and cross your fingers that it isn't something internal. Is your a/c on or off when this happens? I know the weather has been warm out there. BTW: You better post some pics of your car!!!!

Sounds more rotational than the pressure release sound associated with an AOS. Forced to a guess, I'd say alternator bearing or tensioner. +1 on checking with the belt off, check everything that turns by hand if the noise is gone with the belt off. Keep us posted. GL with diag. & fix.

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Check the P/S level first, it's easy to do and either confirm or rule it out. The sound is slightly different than when my AOS failed, but your symptoms are identical to when my AOS failed- the sound didn't start immediately & lessened with acceleration, but was LOUD and otherwise sounded dead-on like the sqeal of a slipping belt. With the engine running and squealing, try to remove the oil fill cap- if it's extremely hard to remove but the sound stops immediately after you remove it, that's your confirmation it's the AOS. (If you find it impossible to remove while the car is running, that's a pretty good indication, too.) Word of warning: immediately put the cap back on, or the additional airflow is apt to suck a lot of oil into your intake manifold through the failed AOS. Which will remove all doubt, sure, but kinda makes a mess and a lot of embarrassing smoke.

wow thats some angry chipmunks

It's the AOS alright. I just got to the gym and The shrieking started up a few minutes ago. I hopped out and tried the oil filler cap test. Sure enough it gave a good fight before popping off with a swoosh like a city bus releasing the air brake and the squeal went away immediately. Thanks, everyone, for the advice! My next question is how long can I drive around like this? She's my only ride so if it isn't safe I'll have to take her to the shop, but if I'm okay for a few days I might try to order parts and do it myself. I can get it up on jack stands and lower the engine a bit, which is what it looks like it will take. A lot of posts say it isn't that hard, but some are saying their mechanic charges for 8 hours of labor! What do you guys think?

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Depends how mechanically inclined you are... The only tricky part as I recall is the clamp for the accordion hose (1-2 in long) that connects the AOS to the block.

You don't want to keep driving your car with a failed AOS. Time to get e rental car and get your car to the shop.

I wouldn't drive it. Get it to the shop.

I can use the girlfriend's Mini for a bit if need be... if I choose to go DIY, what other than the separator (which apparently ships with the boot), do I need in terms of parts? Should I replace any of the vent hoses or lines? I know there's a temptation to replace anything you can reach when you're going in this deep, but what is absolutely necessary?

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Nope, unless you want to replace the (3) driver's side intake plenum gaskets..

Remove serpentine belt temporarily and see if still makes noise. ----------------

hmmmm, where is the noise coming from? by the belts or towards the from of the car.... sounds mechanical to me.

I can see how you would suspect a mechanical issue given the sound from the video. I should've recorded it starting and stopping so you get the effect of the whistling because it definitely comes across as vacuum related in person. Also, there is no question that it is a fight getting that oil filler cap off and the sound immediately stops once it's removed so I'm fairly convinced we've gotten to the root of the matter. As an update, I called my mechanic to get an estimate, just to see if it is worth my time. He got back to me and said they actually have a 996 in the shop right now with a virtually identical sound and they ended up finding a small tear in the AOS boot, which he claims they were able to replace in about two hours. If that's the case, it could be a pretty tolerable tab and perhaps not worth the trouble for the relatively minimal cost. He wants me to bring it in to check out and I told him if it's a $1,700 job I'm going to take a crack at it on my own. Per Ryan's request I posted a picture of the car. Penny is a trooper. She's a true daily driver with plenty of enthusiastic back-road miles, but not an inch on the track. Off the top of my head, I believe her odometer is at 113,000, and a fair chunk of that was earned plowing through Midwestern blizzards on a set of Pirelli 240 SnowSports, passing F-150s and Grand Cherokees like they were standing still. I had her in for some suspension work a couple of months ago and she wasn't losing a drop of oil - no evidence of any RMS or IMS issues, still on the original clutch without a hint of slip, and except for a few shakes and rattles from odd bits of trim, I doubt any layperson would peg her at 11 years young. When I had her down in Missouri, there was nary a creak or groan to be heard and it's only these unconscionably mismanaged Omaha roads that have given her cause for complaint.

just a suggestion.... some guys put on a worm clamp cause the OEM clamp is a bear to put back on.... however, the worm clamp can cut through the thin rubber bellows over time... ask me how I know. Use the OEM clamp. Its worth the extra effort.

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Nebraska roads are horrible.. I used to drive over in Iowa, Loess Hills Byway.. better roads.. Nice car BTW... looks familiar

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Glad you id'd the source of the noise. Damn AOS. My Boxster is on its 3rd one. You can't drive the car much and you really should not drive it all. Not too many minutes of run time after I found my Boxster's oil filler cap impossible to remove against the pressure difference due to a bad AOS the engine starting smoking due to the amount of oil that was being routed to the engine by the bad AOS. My advice is to flat bed the car to a qualified shop and have the AOS replaced. I do not remember the # of hours or total cost, but compared to the cost of a new engine... When the 1st AOS failed, when I called my service manager about what to do with my car he said do not drive the car but have it flat bedded in to the shop. He told me of another owner who continued to drive his Boxster and the engine suffered hydraulic lock and this ruined the engine. So both times I had the car flat bedded in. 1st time the distance from home to the dealer was around 40 miles. The 2nd time the distance from the motel parking lot to the dealer was over 90 miles. Same dealer too. Sincerely, Macster.

He's not blowing smoke, which means no oil in the intake and no risk of hydrolock. Should be low risk to drive, but I wouldn't drive far as it could start sucking oil into the intake at any time. If it does start blowing smoke, it's not going to hydrolock while running, but there is a risk that enough oil could drain into a cylinder after you stop to cause issues. Amazing how little oil it takes to creates a MASSIVE smoke screen! AOS is a ~$100 part and a common repair, with lots of info online. Can't give any first-hand advice though, as it's in a completely different spot on my car. (FWIW, local independent european car parts store had an AOS in stock and took me about 2 hours to change it on my 986S, most of which was cursing at the ^$^#^#$^#@!@#$ hose clamp.)

I've heard and AOS sound similar when there was a low coolant level issue. Did you check the coolant level?

I'm going to lose it! First, thanks for the advice on the coolant. I checked, though, and it's right where it should be. So this morning I set out to the shop to have them take a listen since they just had a car in with the same sound that supposedly needed just a new AOS boot. About five minutes into my drive, just as the engine reaches temperature, the squealing starts up as expected. I arrive early and I know that they are wrapping up with another customer so I park outside the bay and kill the engine. About twenty minutes later, it's my turn before the jury and we fire her up to hear the evidence. Not a peep! I say, "Let's let it warm up..." so we wait and wait. Finally, feeling beads of embarrassed sweat on my forehead, I tell them I'll go wring it for a while and come back when it's up to no good again. I tell you now I thrashed that car like I never have before. I was washing out on on-ramps, chirping the tires at the lights and running it up to 7,000 RPM in every gear. I drove for twenty minutes and it would NOT make a sound! I came back to the shop, let them listen to my video, which they agreed sounded exactly like what they had dealt with on the previous car, and was told to just go about my day and whenever it makes the squeal again to just zip over right away and leave it running. Oh well!

thats why you really need to drive it like you stole it with these cars.... they just work better that way.

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Not now but the onset of smoking can be quite quick. In the 1st AOS failure the engine went from just a mild sort of hesitation to smoking in just minutes of me running the engine in my driveway. There was oil dripping from the exhaust tips as the car was pulled up on the bed of the flat bed tow truck. In the case of the 2nd AOS failure, I admit I drove probably 2K miles with what at the time was a failing or at least an acting up AOS, but I have to say that had I known this, or even suspected it, I would not have continued to drive the car. The only symptom was about once a day, towards the end of the day of driving hundreds of miles at freeway speeds the CEL would come on and the error codes pointed to a lean problem. The DME was at its enrichment threshold. My 'diagnosis' was MAF and I decided to press on thinking that in the worst case I could disconnect the MAF and limp to a dealer. After 2 days of this behavior: once in Flagstaff, once in Albuquerque; I reached my destination (Sedalia MO) and parked the car. Next morning the engine started normal, but there was a cloud of smoke. But it didn't stop. It just keep on pouring out of the exhaust. It finally dawned on me what the cause of the CEL was and I shut off the engine, called a tow truck and got the car to the dealer in Merriam KS. Sincerely, Macster.

http://youtu.be/575JKEOZBNc . Hi, I have read through the forum to see if anyone else has the same or similar problem but as yet I am at a loss and so is my local OPC. Above is the address for the video of the chirping noise and apologies for the erratic videoing. This chirping noise has been going on for months as it was initially suggested by my OPC that it sounded like a pulley on the water pump. Today it went in to the OPC for what was supposed to be a water pump replacement but alas they confirmed this wasn't the problem and they were at a loss to diagnose what other than it was coming from the left bank side of the engine. They ruled out IMS, AOS and Serpentine and suggested that until they got inside the engine and even then no guarantees, would they no what the problem is but so far its a head scratcher. I should note that the noise stops on long runs when the engine is really up to temp and there are no diagnostic issues, it doesn't use much oil and touch wood, runs sweetly. Any help suggestions would be greatly appreciated. Thanks, D

Hello again, all, and dab996, A new potential culprit has come to light. The techs at Woodhouse Porsche were as perplexed as I by the seemingly unpredictable onset of the noise. They were unable to replicate the banshee wail I captured on video, but they did hear a noise with the car running up on the lift and tried to locate it. As we talked about the conditions under which it emerges, the only possible pattern is that it seems to happen only on the first drive of the day. I can set out for work and after a few minutes I'll hear it screaming away, but then at lunch and again on the drive home, silent running. That has led us to speculation that it is the secondary air injection pump or an associated valve. Since its primary function is to assist with cold starts, that may explain the tendency to only crop up after long periods of rest. I still say that the amount of vacuum at the oil filler was unsettlingly high, but without a manometer to put numbers on it, I can't really say. I'm going to figure out which relay will kill the SAI and see if the sound appears again. I'm sure it'll throw a CEL, but we'll know the culprit so that's fine. If I don't hear anymore squeals then I'll replace it. It's mercifully accessible through the deck-lid. dab996, your sound could be vacuum or exhaust related. I read a thread over in 6speed with a description of a similar noise that ended up being a leaky exhaust manifold gasket. Do a search. The poster was MrWhite. I can't imagine that they didn't do something as simple as running the engine with the belt off to rule out a pulley, but it's easy enough to do that it may be worth a shot on your own for peace of mind because it sure sounds like it. Video can be deceiving, though.