I am going to plead ignorance and ask if the alternator is new (if it's used it could have a bad bearing) but it still sounds like the belt tension isn't right. Get someone to start it and watch the belt - it might give you a clue on what it's actually doing. Maybe try cleaning the pulleys as they might be contaminated. If it's having some kind of load problem it can cause the alternator to bug out (small alternator + big audio system = bad)