First post, I hope in the right place!

2000 A6 Avant Quattro (AWD wagon)
Tiptronic
85k miles
Excellent shape and clean history

Here's the riddle... I get a whistling noise (sounds like air escaping from a high pressure hose) under the fllowing conditions:

Car is warm (will not make the noise when cold)
Approx 30+ MPH
I am in gear, under load (i.e. it wil NOT make the noise on a rack)
At speed, when I remove foot from accelerator and allow the car to "coast" while in drive
While I am coasting andhearing the noise, if I slip into neutral the noise immediately vanishes.

Given the above, I believe this must be related to the transmission, however, the car is running perfectly, i.e. no transmission slippage at all.

The noise is loud enough to hear inside the car. Louder the faster I am going when I take my foot off the gas.

Any ideas??? Thanks.

 

Take off the serpentine belt after the engine has warmed up and take it for a drive and see if its still there. I've found several whistling noises from the serpentine belt tensioner/pulley that sounds like it could be a exhaust leak or transmission whine.
Its easy to do so give it a try.