I have a 1997 Pontiac GTP has slack on supercharger belt and the tensioner is clicking back and forth. Belt is the right size. Already tried changing the belt with a smaller belt.
It sounds like the tensioner needs to be replaced as the internal spring is to weak. The only thing would be to make sure none of the pulleys the belt rides on are sticking/hanging up. Spin them with the belt off to check them. There is no adjustment for the tensioner and they commonly need to be replaced. They probably have one in stock at your local auto parts store.