Driving my 2001 S4 today I heard what sounded like a motorcycle following me at a distance. The sound turned out to be from my car. It isn't an exhaust noise because it is related to speed rather than engine power. At 30-40 MPH it is like the rumble of a motorcycle. At 80 it is higher pitched, a low pitched whine. (I didn't think to check what it sounded like at 120.)
It's from behind and to the right of the driver's seat. It sounds about the same with the sunroof open or closed.
Any idea what this might be?
The car has 66,000 miles on it. The rear differential seals were replaced at 48,000 miles, when the dealer did a detailed inspection for the last free service.