Hey Everybody,
My 99 A6 Avant with 2.8L Auto trans, is making a clicking or rubbing kind of noise only when I accelerate. To make matter worse it only happens when the car has been driven for a while, the engine is nice and warm and the weather is around 70. It does not do it when the weather is around 60. The sound is only there when I accelerate, not when I idle or coast.

Here is a youtube video of the sound.

http://www.youtube.com/watch?v=nSBk_pJaXL8

Ideas so far, Differential fluid flush, CV joints (just had the front end checked by a mechanic and nothing came up)

 

Do you have any misfiring problems? If yes, then check your spark plugs.
If you haven't misfiring problems, then it could be one of the following two:
- Dust shield rubbing against the rotor
or
- a Pulley
(However, my ear isn't that mechanically inclined to detect a specific noise via a video clip)

 

I don't have any codes or any lights on. The engine does not do it when I rev it at idle. The sound seems to come when the weather is warm and that is fudging weird. Will check the rotors and stuff to see if there is any rubbing once they get warm.

 

COuld be A/C related - pulley or secondary fan. You only here it at higher RPMs on a warm day...

 

If it is indeed temperature-related, then most likely Bob's got it.
In that video clip did you have the AC running? Or a better question: is the noise exhibited mostly when the AC is on?
If it is, then the simplest way to determine if it is AC Compressor-related is to actually drive without the damn thing on to see if it goes away.
------

 

I will have to test it with the air off. I did turn the fan on. Any ideas where that secondary fan is located (i will google and check the A/C pulley). Did I mention that going through a car wash last night made the sound go away for 5 minutes.

Also can we rule Differential, and CVs out?

 

I'd investigate on the cold air intake, not excluding the filter and the MAF.

 

Does it sound like it could be exhaust leak at all. Usually its the other way around, loud when cold and quiets when warm. Just a thought though.