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".

 

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