Probally the tensioner or a loose/warped pulley. Remove the belt and start it up and if it doesn't make the noise anymore you know it's in the rotating accesories. While the belt is off, check for play and spin them by hand and see what you find.
agreed. the bearings in the tensioner pulley can go south. i could hear mine when i removed the belt and spun it by hand. try to get a metal pulley (IMHO) if you need a new one. i am not fond of plastic ones.