need help from a wise sage- wierd snarl/buzzing sound on hard accell,

[p0rsch3]

ok so this is bizzare and it's driving me nuts. I have this odd buzzing-snarl-rattle coming from somewere in the rear engine area whenever I floor the car from a stop or slow speed in 1st.

This only happens when the car is under full load / WOT in 1st and i'm either stopped or practically stopped, and it is most prevalent at the lower end of the RPM range when car is pulling hardest to move the weight of the vehicle. It is very consistent and I can make it happen on command, drawback is the vehicle has to be moving which makes it tough to stand outside the car and troubleshoot...

If I rev the car in N it does not occur. If I floor it in 2nd or other gear it does not occurr.

I thought it might be a bad muffler so I just replaced them, and it is still happening. It is not the tips rattling either.

I thought it might be the intake (K/N) kit and checked it for looseness, all looks good.

Is there some sort of diverter valve or something that could be making this noise under the described conditions? It's a wierd buzzy-resonant-rattle sound that has a hint of mechanical to it.

i'm ready to call a priest and sacrifice a small goat (...and not neccessarily in that order)

Edited March 1, 2008 by p0rsch3

[Loren]

Have you checked the muffler heat shields?

[p0rsch3]

Have you checked the muffler heat shields?

Yep, I had the entire back bumper and shields off yesterday when we did the muffler swap. Nothing loose, broken or (from what I could see) rubbing. I also did some slight shield tweaking just to be sure, because this was my initial thought as well.

The odd thing is, if it were simply a resonant vibration it should also do it when revving in neutral or hard accell in other gears correct? It only makes the sound when accellerating hard from a stop or slow roll with wide open throttle in 1st. If I do the same in 2nd gear from stop or slow roll, no noise.

[1999Porsche911]

Could be belt or a pulley. Quickest way to test is to remove the belt and try it. If there is no noise, then change the belt. If noise comes back with new belt, it may be the tensioner pulley. Costs about $60.

Obviously, do not take a long leisurely drive without the belt on as you will have no water pump or power steering function. Test it on a cold engine.

Edited March 1, 2008 by 1999Porsche911

[p0rsch3]

Could be belt or a pulley. Quickest way to test is to remove the belt and try it. If there is no noise, then change the belt. If noise comes back with new belt, it may be the tensioner pulley. Costs about $60.

Obviously, do not take a long leisurely drive without the belt on as you will have no water pump or power steering function. Test it on a cold engine.

worth considering, but sounds kind of dangerous to test like this- is there a DIY here on removing belt and location of tensioner pulley?

[wvicary]

Have you checked the engine/transaxle mounts?

[p0rsch3]

Have you checked the engine/transaxle mounts?

Not yet, but I think I may need to replace one of them- I have a definite 'thump' when hitting the 1-2 shift hard, which feels / sounds mount-related.

could the vibrating sound be related?

[wvicary]

A bad mount puts the whole power train out of whack and could be the cause of the vibration but from your description it sounds exhaust related. I was thinking along the lines of the exhaust system being too close to a suspension component or heat shield because of a poor mount.

[p0rsch3]

A bad mount puts the whole power train out of whack and could be the cause of the vibration but from your description it sounds exhaust related. I was thinking along the lines of the exhaust system being too close to a suspension component or heat shield because of a poor mount.

Yep, that was my first suspicion as well- but after having changed the mufflers and disassembling the bumper / heat shields for a thorough inspection it did not cure the problem.

The thing that makes it so darn perplexing is it only does this under full throttle in 1st. So maybe.. if the mounts are not solid the drivetrain is flexing from torque and causing something to touch / vibrate?

Is it tough to replace engine and tranny mounts on a (tiptronic) pcar? Have done this on other cars but not on a Porsche.

Edited March 2, 2008 by p0rsch3