Ok, at speed with foot on gas, there is a pretty good whine. If you let off the gas, the whine goes away. A4 tranny in a 02 z. Any thoughts?

 

same problem with mine. Guess where mine is now. In the shop getting all new bearings in the rear. But I am also getting a 3.73 installed so I am pretty happy . But you may want to get that checked out asap.

 

If the sound is coming from the rear axle then it could be the way the ring and pinion have been set. The pinion depth and back lash have to be done correctly to set up a quiet differential. This is done with a crushed sleeve and side shims. It is then checked with marking paste for pattern.

If this sound has just now started without any work having been done to the car then this is probably not the reason. Welcome to the forum.