Hello Martynas,
I would very much agree with Trevor's advice about checking the belt and it's assembly. Just one further point I would add, the previous owner may well have had the water pump changed at the same time as the belt and tensioner. Unfortunately aftermarket water pumps ( if fitted at the time) can be of highly variable quality and service life, and if it has survived for 20k then you are doing well!
Appreciating the expense, but it may be well worth doing this again and investing in a VAG belt and tensioner and water pump (not aftermarket pump). In fact, I pretty sure Audi do rebuilt pumps on an exchange basis - or they were. If you have to go for aftermarket belts etc. then the only one I would go for would be Gates. Apreciating Gates do a kit inc. water pump, but for me it would have to be a VAG pump to go with their belt kit.
Of course, having said all that, it may well not be the timing belt/water pump assembly. Might be worth removing the auxiliary belt first and seeing if the noise disappears. If so, the noise will be associated with whatever it drives- alternator, P.S pump etc.
Good luck with it.
Kind regards,
Gareth.