This noise wasnt their before the alternator change? The replacement alternator is a good used unit right. Im going to guess the noise is something in the alternator making the noise ,A washer or screw has found its way into the unit,If it sat around on a bench or a parts ben this could be a very possible scenerio .It also could be its bearing going out.You could take it up to a parts store to have it bench tested while also hearing for this noise to make itself known.If good but only noisey take it apart to see if something is in its cage. If not its likely a bearing issue