I dont know what car you have exactly, but on my mk3 when i had mine done, it did exactly as you explain, started getting very noisy, when it was warm and above like 2000 rmp. i took it back and it was explained to me that often, because of the way the tensioners work on VWs the belts are set too tight when they are first done and loosen up after an average of about 500 miles. mine is now fine and makes no noise at all.