I can't diagnose the actual noise coming from the alternator, but the noise coming through the stereo can probably be solved with an Rf choke that Radio Shack sells just for this kind of problem. As too the alternator itself, is the sound recent, is it constant over time(IE: over a couple of weeks) or is it getting louder? If it's getting louder, I would replace the alternator. If not, I would simply keep an eye on it. Are you getting enough voltage out of it?(14v+) if not, something in the electrical path is crapping out, otherwise my suspicion is the bearings.