One problem related to engine grinding noise has been reported for the 1998 Audi A4. The most recently reported issues are listed below.
Multitude of probs. W/ almost every aspect (brakes, engine valves, oil light stays on, rain leaks, bad wheel bearings, etc. ) of this car. Most frustrating has been rude and incompetent service by Audi dealer. After an expensive repair to the rear wheel bearings the dealer said that although not all the noise was gone, the car was safe enough to drive to calif. ,(we live in NJ). We did not believe the dealer because a loud grinding noise remained and took it to another shop where both front control arms were found to be damaged. We were advised not to drive the car as it was not safe to do so .