[rallyingrob]

could be bearings starting to let go.....
if its still charging properly, then it isnt (likely) a winding or something coming loose

it could even be a piece of debris caught in the alternator somehow.......did u ever have a belt break?(including a/c) a piece of the old belt could be in there....i had that with mine.....no noise or charging issues due to it....but pieces of belt literally hanging out of the housing...