Hi Gareth,
and thank you for your suggestions. Since the sound is intermittent, I find it difficult to search for the source. If I, as you suggest, remove the belt and start the engine, I'll never know if the sound is just at "pause", or if I have actually found the source? Unless, of course, the noise starts becoming constant. I could, however, remove the auxiliary belt to inspect it for any damage or defects.
The car has done 107 000 km (66 486 miles), and I don't think the timing belt, water pump or the auxiliary belt has been changed earlier. These items are, however, due to be changed at the next service at 120 000 km, which — with my driving — would be due some time next spring.
The car has been at the local Audi dealer for a brake change a couple of weeks ago, without them finding out anything about the noise (as it, of course, wasn't there when they had the car).
Best regards,
Sindre