umm, no one has suggested this yet...you should inspect your belt first, make sure there are no cracks or any wear in the treads. Then turn the car on and look down there, make sure the belt is moving in a straight line (no wobbly pulleys). If everything checks out it's probably the belt tensioner.
My cousin had this problem on his GTS, we inspected everything and it was fine. He brought in to the dealer, they replaced his belt tensioner under warranty, problem solved. And he didn't spend 20 bucks on a belt that would've done nothing for him.