Hmm that is a viable culprit, and would normally be load related as the prop does move about a bit between acceleration and overrun. I've no experience of how Quattro behaves when that bearing is worn, but I do have an A4 prop in the workshop with a very loose rattly bearing so its definitely possible for them to fail like that.
IIRC you can't actually see the bearing on the D2 as it's hidden by the heat shields. To get to it you would need to remove the exhaust system and obviously the heat shields and then you can inspect and / or drop the prop. If the bearing is worn enough to be noisy its likely you'll be able to feel some play in it.
Typically Audi, you can't actually buy the bearing separately as the prop is balanced as an assembly and the bearing is non-trivial to change. People have found solutions for the smaller models, but I'm not sure about the D2. Its easier to just change the whole prop for a good used one.
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