alternator hum

Hi replaced uim/lim gaskets on daughters 2001 monte ss 3.8 and everything seems to be fine , runs good but I noticed when car is off there is a hum from the alternator what could this be.I cleaned battery connections and checked things over and over.I think it was fine before I took it apart.

Are you sure it's from the alternator? If yes, then what I would do is remove the battery cable from the back of the alternator to see if the hum still persists... go from there.. Let me know the verdict.