Owners of 2004-2009 Audi A4/S4, 2005-2010 A6 and 2007-2010 A8 vehicles may report vibrations, humming noises or rubbing noises from the drivetrain when driving slowly around very tight turns. The noise is particularly noticeable when maneuvering at full steering wheel lock in a parking lot.
Friction-reducing additives in the differential oil are reduced by wear. At high load on tight turns, the friction discs and worm wheel teeth in the center differential are subject to wear. This changes the friction and locking values in the center differential and causes vibrations in the drivetrain.
Test drive the vehicle. Light vibrations at full steering wheel lock are normal. With stronger-than-normal vibrations in minor corners or heavy vibration at full steering wheel lock, proceed as follows: