Did this start right after the new tires were installed? The serpentine belt has an automatic tenshioner and can't be adjusted. Only replaced. I suspect if the belt was loose it would squeel when you turned the wheels. Unless, of course, the people in the shop put some oil on it to make it slip?? Do they do general repairs too? Sorry to be so suspicious but my son's girl friend just went throught hell and $550 dollars of repairs after taking her Passat in for some "routine" recall work. Try a new belt and clean up all the pulleys and rollers.