Sounds like the belt tensioner. Could also be a bad waterpump bushing, alternator bushing, or AC compressor clutch. Most of these you should be able to diagnose with your hands with the engine off. If you grab each pulley (or tensioner) real tight and shake it, it shouldn't move.
Another way to check these is to release tension on the serpentine belt and start the car. If the noise is gone, it's one of these things mentioned above. Don't let it run for more than a minutes or two with the belt off or it could overheat because the waterpump will be essentially disconnected.
Wednesday, November 14th, 2012 AT 8:33 AM