If the bearings are worn... they can make an awful noise....Seems a little unlikely to me, but I'd just check to see that there isn't too much play on the shaft by grabbing hold of the pulley and pulling outward away from the alternator. If the shaft and pulley start to slide , then it may be time for new bearings, or a new alternator.
Otherwise, I'd check belt tension. ...:thinking:

Good luck.