Sounds quite odd. I was going to say that it sounded like a slow or dead water pump but that would then not have the hot air and would overheat more I would have thought unless there is just slight water movement. I think that the mechanic will find it and I would not be surprised if it is just the water pump and / or maybe something that controls the speed. On my TT I have the normal water pump running off the engine and then another extra electric pump that is electronically controlled.

Good Luck