One problem related to engine has been reported for the 1997 Audi A4. The most recently reported issues are listed below.
I was told by the dealer that the control arms are failing because Audi didn't provide for lubrication points. . They had no explanation for the loss of the engine mount bolts, and could not respond when I said I had never owned a car that had lost it's engine mount bolts. . Obviously this has also got to be a problem caused by poor design and engine vibration. . These bolts should be safety wired to prevent this problem. I think Audi should cover the costs of replacement of the control arm and engine mount, but the differential problem could be due to wear, although again I don't have this problem on my Jeep or other vehicles.