996 M96 engine noise at idle.

Good afternoon, I am currently trying to track down the source of a rattling noise in my 996. I recently removed the engine to replace the coolant tank, air oil separator, spark plugs, starter cable, IMS, etc. For the IMS I ended up just removing the outer bearing seal per LN Engineering's recommendation as I found out my engine was exchanged through Porsche somewhere around 6 years ago and has upgraded 997 parts on it including the larger bearing. I have driven the car and it drives fine. I thought it was the mufflers or catalytic converters, but the noise is still present with the mufflers off and the cores appear to be in place in the converters which rules those out. The noise is only present when below 1k RPM. At or above 1k RPM and the noise completely goes away. Once I let off of the throttle and the RPMs rest back at idle the sound comes back. It also makes a noise when shutting the car off. Here is the video where I start the car, increase RPM above 1k and then let it rest at idle again before shutting it off. Here is the video where I increase the RPM to about 4k RPM then let it rest at idle again.

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Loose chain tensioners? (Says the guy who isn't definitely NOT an expert on these things...)

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Haven't listened but clutch release bearings are common sources of idle rattles; have you checked it?

Sounds like your DMF has failed, guess you didn't test it while it was out?

Have you tried running without the serpentine belt (for only a minute) to see if the nose is present? Could you possibly have a hose bumping up against anything?

I have a fuel vapor canister solenoid on my engine that makes a ticking/tapping noise. It only does it when the vehicle is fully warmed up and goes away above 1500 ish rpm. The solenoid can be found above bank one, behind where the air intake snorkel meets the rear wing. I can feel the pulse of the tapping noise when the noise is present. This might not be the root of the noise you're hearing, but it's worth a look.

Post a video of it and others can help identify said noise.

I just found the skid plate on my 996 lose, it was making a very strange rattle when in idle.

I should have mentioned that this is a 2003 C4S Tiptronic. So no manual transmission. I started it without the serpentine belt and surprisingly the noise wasn't present from what I could hear, but there was still a rattle when shutting it off. Here is the video running without the serpentine belt. Here is the video running right after putting the serpentine belt back on.

Hand spin all pulleys (alternator, water pump, idle pulley, etc.) and feel for rattle, noise, etc.

Still sound like something loose in the flywheel / torque convertor area( the rattle on shutdown).. Did you have any problems bolting the convertor back up?

I spun all of the pulleys by hand and they all spun smoothly. The water pump is brand new. I removed the engine and transmission as a unit and them separated them. When I did the torque converter slid off the last set of splines and maybe half of a liter of transmission fluid leaked out, but I definitely installed the torque converter back correctly by having all sets of splines aligned because I was able to lock the converter through the hole in the top of the bell housing. To answer your question, no, I did not have an issue at all bolting the torque converter back to the flywheel. The only possible issue I see is that I reused the bolts for the flywheel and was not sure of the torque spec so I may have under torqued them. I reused the bolts for the torque converter, but the torque spec on those are so low I don't think it would make a difference. Best regards.

I read the flywheel bolts are one time use stretch bolts?

Correct - flywheel bolts should only be used once. Get yourself a copy of the workshop or Bentley manual or even the flat six innovations torque specs book IMO a torque value is essential

They're grade 12.9 M10 bolts, at the torque spec prescribed by Porsche they are nowhere near yield strength (25NM initial + 90 degrees). Afaik that was the first revision from a straight 67 ft lb recommendation in the early manuals. Nowadays I hear 25NM + 120 degrees being thrown around as well. Anyway, recommended normal torque (not torque-to-yield) for 12.9 M10 fasteners is 63 ft lb or 85 NM. 25NM + 90 degrees ended right around that mark (I did mine a few days ago). tl;dr the flywheel bolts are not Torque-to-yield / stretch, thus can be reused.

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Regardless the factory manual states to replace them. They aren't that expensive, why take the risk if you're in there already? Just my opinion

Flywheel bolts should be 19 ftlbs + 90�. That’s actually not a lot of torque.

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I mis-quoted M12 numbers in that post, sorry about that. The 25NM+90deg stated in my owners manual comes down right around the regular torque mark. If you did 120 degrees, you might be stretching them. They're cheap, sure, but sometimes not readily available when you're working on the car. I'd have replaced them if my dealer could've supplied them. They couldn't, so I reused mine. I can't seem to find the part in the manual that states new bolts should be used though. Which manual are you talking about? I've got the Porsche Technical Manual.

Here’s the Bentley Manual page (30-5) on flywheel install: While it’s not a factory Porsche manual, I’ve found Bentley Manuals superior to most manuals, including Volkswagen factory manuals.

Alright, while it says that, the basic physics say it's not absolutely required if you can't get your hands on new ones. Applied correctly, they are not stretched or torqued to yield. New ones can't ever hurt if you don't know the exact history of the car, but re-using properly torqued ones will be absolutely fine.

You’re right, it will probably be ok to reuse them. But if it were me, I’d buy new bolts when I purchased the new clutch. $16 for new 8 bolts would be worth the peace of mind.

They�ve also got the green coating on them, so there�s that... Did you use any blue loctite or similar? I�m not saying they loosened up... just throwin� Ideas out there - good luck

I probably should have replaced them, but I didn't see much of a reason to. They were in great shape and after measuring them they didn't appear to be stretched at all. I did use blue Loctite. I do on nearly everything that will experience vibrations. I did torque them to the correct spec. I wasn't sure if the 19 ft/lbs plus 90 degrees was correct as I had read 120 ft/lbs somewhere else, but that must have been for a DMF. I am not concerned about the bolts now. Still trying to narrow down the noise.

Have you tried the spark plugs yet? Bit of a PITA to get too with exhausts etc in place, but feasible.

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I did replace the spark plugs. Are you thinking they are not tightened all of the way? I had never changed spark plugs on a Porsche, but have on many BMWs and never had an issue. It has crossed my mind that they were not tightened enough and could be leaking. I will check that. I bought new bolts and flipped them for the exhaust so I could easily remove them and have done so several times already when looking for the rattle.

I tracked the noise down to 100% being the torque converter. I'm not quite sure if it is damaged or not. As mentioned in my first post when I separated the transmission from the engine some fluid leaked out of the torque converter. After putting the engine back in the car I started the engine several times and moved it around my driveway. It wasn't making this noise originally. Today when I finally realized it was more than likely the torque converter I went and started the car in neutral and it was dead silent. Put it in gear and immediately started hearing the rattle. The transmission pan gasket had a slight leak so I went ahead and replaced the gasket and filled the 722.6 transmission with the proper fluid using the Porsche tool through the fill port. I performed the proper procedure and the transmission took a little over four liters. Even after all of this the torque converter is still making the same exact noise. Is there a possibility that it was damaged when I started the engine?

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I have seen a few flex plates/flywheels cracked or broken. I'm not going to guess as to what mite have caused one to do that, but you may have to pull it back out to see, or see what other may have happened..

I figured out the problem. When I changed the fluid in the transmission a couple of days ago I could see the torque converter wobbling through the two windows on the bottom of the bell housing where rubber plugs go. Last night I started prepping the car to remove the engine again. Before I went to bed it crossed my mind that the M8 bolts for the torque converter could be loose. I remember putting Loctite on all of the bolts on the car except those for some reason so I figured I might have forgot. Today I left the engine in the car and removed the front intake plenum along with the starter. At first glance the bolts looked tight, but when I touched them they were loose. All six of them. I had forgotten to put Loctite on them. I put blue Loctite on them and tightened them back down. After reinstalling everything and starting the car the rattle is gone. This would explain why the rattle appeared after starting the engine several times. I didn't hear it the first few times the engine was started until the bolts started to back out. Hopefully this helps someone in the future track down a rattle on their own 996. By the way, according to the Porsche Workshop Manual it appears that it is alright to reuse the old bolts for the flex plate on a C4S. New bolts are to be torqued to 19 ft/lbs + 120 degrees. Old bolts 19 ft/lbs + 90 degrees. It says a C4 is 90 degrees only. I have attached a screenshot for future reference. Best regards.

Good detective work!