- An '03 Clio 1.4 ; the cushioned pulley was breaking apart. I thought the head was going! - A 1.8 Ford Mondeo '07 Mk3 ; the one way clutch on the pulley had seized and it sounded like the big ends had gone on the engine! - My '03 M3 ; the bearings/fan were not too clever and they began to rattle ; that did not sound nice!
On the latter the alternator bearings are not the worst I've had on an engine but the idler pulley had seen better days ; that was shot! It's alot quieter now with new belts and pulleys, oh, and alternator; the latter decided it would become an intermittent charger.
In your case I would remove the aux belt and see what happens as a surefire way to diagnose it. Then feel the pulleys one by one to rule out each component.