cal45

Bought my 2003 A4 coupe in Sept 09 with just 4,500 miles - now has 6,400 miles. I notice that when I shift the car in reverse with the brakes on, there is a vibrating noise that seems to be coming from the engine compartment. Nosie will decrease & sometimes stops when the car starts to roll and it does not make the noise in Drive (moving or not). Anyone have any ideas what this may be?

Thanks