I don't know what the bottom ends sound like when they go on these, but I would say it was worthwhile checking all the belt tensioners & pulleys - there are quite a few, I would expect them to make a similar sound.
Im not an expert on the 2.5 tdi in trhe Audi but on the 2.5tdi T4 a similar sound would occur when the vibration damper pulley was disintegrating. ie rubber inner starts to break up and if it goes it takes the cambelt and everything with it. Which is my way of saying get it checked whilst its not costing you anything. As the pulley wears the alignment also shifts and the belts start to rub or catch. I could also be talking rubbish but that was common on 2.5 tdi after about 120k.