Mysterious Buzz

My 2000 996 cab has a mysterious buzzing that occurs only during hard acceleration at high revs. The noise is like sheet metal resonating seemingly around the drivers side wheel. It seems to occur around the same time resonance flap opens, or around 5500rpm to redline. It never occurs below these rpms. I've attached an audio clip. Does this sound like heat shields or something else? The noise appeared after replacing the IMS bearing, clutch, DM flywheel, and adding Tubi exhausts about 3K miles ago. Banging on the tips and cats produces no substantial buzz or rattle. I've driven without the tips, and the noise remains. You can rule out the Tubi's as I've already replaced them with a new set and the noise remains. My mechanic suspects the Tubi's are making other areas of the engine bay resonate at certain rpm. He may be right. Thanks in advance for any comments.

I did not listen to the zip file - because I'm paranoid like that - but a buzzing sound from that area will generally be: * Loose heat shield * Chunking catalytic coverter * Loose exhaust bolt/nut/clamp buzzing in place If the issue was due to the mufflers, you'd have one specific rev range where it was especially bad as you hit the resonant frequency for the exhaust. If it's happening at a wider range of RPM (which is how I interpret your description), my guess would be one of the three items enumerated above.

5CHN3LL, I doubt the mufflers are at fault as I've gone through two pairs. The buzzing occurs from 5500 rpm to redline. The audio clip is a zip file because you cannot upload wav's or mp3's. What other methods are there?

Curiosity won out over paranoia, so I gave it a listen. Maybe it's just my crappy speakers on this PC, but I couldn't hear the rattle. I could hear your Tubi cans, though - sounds just like mine ripping through the gears. Love it. Give your exhaust piping a gentle whack with a rubber mallet. If it's the heat shielding, you will likely hear it rattling. If it's a catalytic converter breaking up, this is generally more pronounced when the car is warm; mallet whacking may not yield any results. Both of these are incredibly common - which sucks, since the replacement cost for the cats almost always includes a comma.

I appreciate the curiosity. Yes, the Tubi's sound awesome, but there is a very noticeable buzz in both gears at high revs that attempts to overshadow them. Both Tubi and my mechanic picked it up right away. You might need nice speakers or headphones to hear it. I'm not sure what a failing cat would sound like. Is that a noise that usually manifests itself at high rpm? Also, I've never thrown a CEL.

A failing cat sounds exactly like what you initially described - a metallic buzzing noise. Imagine a bolt sitting inside one of your exhaust pipes, buzzing and rattling. Your location - the driver-side wheel - is where you find both heat shielding and one of the catalytic converters, so while I won't bet you $100 that one of those are the culprit...I would put $5 on it.

I would also suggest taking a look at the heat shield. I had a similar issue and it was after installing Fabspeed exhaust. The shield attachment can rust out over time. The new exhaust created more vibration which, in my case, made the noise more noticeable.

Thanks for the feedback. It's more like metal on metal buzzing than a rattle. I'll pull the rear bumper and poke around at the heat shields. The edges of each heat shield have insulated tape around them that appears to be breaking down. Is this from the factory? I'll start by replacing that and checking all the connecting hardware for corrosion. I'll report back if I find anything.

Thanks for the feedback everyone. This past weekend I had a chance to get under the car. I had previously banged on both cats (cold and warm), which appear to be in good shape with no rattles of buzzing. I removed the bumper, exhaust, and all heat shields. All appeared to be in good shape with no rust and no obvious rattles or loose buzzing. Regardless, I taped over any areas that come in close proximity to any other metal or plastic with insulation, see here: http://www.jegs.com/i/JEGS+Performan...HYgaAtqV8P8HAQ The edges of every heat shield had already been wrapped in a sort of metal wrapped fiberglass tape, which I assume is from the factory? After removing the exhaust brackets attached to the muffler, I noticed one of the bolts seems to have been re-welded on at some point, and the metal had warped slightly in that area. This left a small gap between the two thin sheets of metal. Banging on the bracket with a rubber hammer produced a familiar rattle. Because I didn't have the tools to weld these two pieces together, I placed a piece of insulation in the gap which prevent the two piece of metal from touching. After re-installing everything and going for a test drive, this does not appear to have made a noticeable difference. Thoughts?

Do you have older or newer Tubi mufflers?

I think it's still likely your cats, especially if you've done the work to identify that it's not the shielding. "Mysterious buzz" reminds me of how I felt after a Robert Palmer concert in 1990.

It's a 996.1, so probably older Tubi. Plus, I've already replaced them once so we can rule them out. By the way in no way am I trying to knock on Tubi's or Champion, I've seen nothing but 5-star service from both. 5CHN3LL, I'm glad you found the thread to be "simply irresistible".

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Why did you have to replace them? Some of the first gen Tubi's had some defective internal baffles that would come loose, hence the reason why I was asking.

Yes I suspected something was off with the first pair, mechanic who installed them agreed, and they were replaced at no charge. The newer pair is much more crisp, and less bellowy at the low end.

Could it be the transmission? Any difference in shifting? U say fw was changed was clutch/pp changed too? Does it happen at idle while revving to 5500 rpm? If so have a friend listen from the back to locate the buzzing. Mike

That's a good point about the transmission. Fw, clutch, and pp were all changed. The rattle does go away or lessen when the clutch is in. You can hear this in the clip I think.

I take it the gear oil was changed too? Maybe it needs some breakin miles but hopefully nothing wrong with the synchros. No balky shifts rt? Shifts like before. Buzzing can be an early sign of synchro wear. Hopefully it's something else simple. Mike

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It has been over 5k miles since the change. Car drives and shifts like a dream. No complaints there. Plus, only 60k on the odo.

So gear oil change 5k miles ago or all the internals done 5k miles ago? Maybe change the gear oil and see if old oil has any metal shaving residue in it to rule that out? Mike

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I see what you did there. Nicely played.

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The fw, clutch, and pp were changed 5K miles ago. I do not believe the gear oil was changed that that time. Really, metal shavings in a transmission can could cause a noise like this? I hope it's something that simple.

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Here's a soundcloud link to a few recordings. No zip files https://soundcloud.com/user34598673467454 I've been told the tinny sound at the top end is not normal. That is the "buzz" I'm referring to. Keep in mind that in these clips I am basically flat out trying to reproduce the noise.

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No that's not what I meant. Metal shavings can indicate something wearing inside the transmission ie synchros etc and that affected part maybe the cause of the buzzing. Mike

Sorry, but I do not hear the noise you mention. But if the noise is in fact present and appeared after the work you listed in your 1st post there's probably something -- a line, heat shield, exhaust system component -- something touching something it is not supposed to touch. IIRC not too long ago -- within the last 3 or 4 weeks? -- there was another post on a strange noise that the owner finally traced to something making contact with another part of the car. The noise appeared after he had some work done on the car, too. I have not had loose heat shields with my cars but the Boxster had a buzzing converter. The noise was very noticeable at low speed low RPM operation like driving around in a parking lot in 1st gear at barely a walking pace. Upon accelerating in 1st gear the buzzing remained obvious until at some higher engine RPMs and faster speeds the buzzing became lost in the general noise of the car. Thus I do not think what you are hearing is a bad converter. This doesn't rule out some kind of mechanical failure inside the exhaust system though. My advice is to carefully review/inspect the work done looking for some line, wiring, exhaust system, even heat shield, not routed or reinstalled correctly.

Another question is noise reproduceable on every gear at 5500 rpm? Mike

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Yes, I can verify the noise for 1st, 2nd, 3rd. Cannot verify 4th as that requires a little more road and/or a track... In the clips posted earlier it can be hard to distinguish between the Tubi's and the metallic sound. It's most present in the second clip in between the shifts from 1st to 2nd gear. It wasn't until I started searching around for other folks' sound clips, that I noticed something was different about mine. One other thing to add is that it only occurs on WOT. If you apply the throttle slowly with less load on the engine, it doesn't occur. Btw, I searched this forum and others for months regarding this issue. There have been several threads in the past describing something similar, but none were resolved. I appreciate the feedback so far.

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Listening to the clips again, it occurs most noticeably in "Audio sample 2" at 2.5 seconds and 5.5 seconds. The rough metallic sound in between shifts might be a clue, unless that's normal too? FWIW, any non-Porsche person who takes a ride notices it right away and will ask about it; so it's not just me. Also, I've driven other 996s with different setups that do not make the noise. All in all, so long as it's not killing my car I can probably live with it. But if it's a simple fix I'll go in and get it done. See post #9. I haven't looked for any loose lines or wire, particularly around the transmission.

Do u have a hx of hard launching 1/2/3 gears- good in some ways : ) bad in some too : (. Mike

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Nope, I rarely launch.

I suggest at a minimum change gear oil to see if it quiets down. If so and then it starts again something transmission related is going on. Mike

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This is a good point, especially considering the car is 14 yrs old with 60k miles. However, if gear oil is the culprit then it is my understanding that the symptoms would then be whining at speed and difficult shifts; I have neither.

Buzzing or early gear whine is an early sign- the balky shifts come later. Not saying you have a tranny issue but try the gear oil change. Maybe thats all that's needed. Mike

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I'll definitely give this a shot. How safe would it be to remove the muffler heatshields for a short drive, just to rule them out?

The thought of a melting bumper comes to mind. I probably wouldn't. I probably would take off the whole bumper and heatshields all together and drive it bumperless. Looks ugly but u find out if it the cause : ). Mike

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+996