A6 Quattro rear diff problem

[Raven Flyer]

I put my car up on a friend's two post ramp yesterday as I heard a whine when using the car with the rear seats down, a couple of weeks ago. The rear diff made a terrible whining noise when run up in drive.

Is there a bearing that can fail or are the diffs a known problem area? There are no oil leaks from it and the service schedule would seem to indicate they are sealed for life.