Chevy Lumina-Sedan 3.1L, RPM NEEDLE OSCILLATING SLIGHTLY UP AND DOWN, at Idling. on 1995 Chevrolet Lumina

I recently notice that when my GM Chevy Lumina-Sedan LS 3.1L is idling, its RPM guage needle oscillates up and down slightly and not too fast like vibrating. Also, there is a humming noise from the engine compartment.

I want to fix this problem before it gets worse. Your advice will be appreciated.

Asked by for the 1995 Chevrolet Lumina
Does the humming come and go as well and at the same time as the Idle changes? How much RPM variation is there?
Thanks for your reply! Yes the humming noise come and go as well, it was around the alternator.
RPM was about 900-800, which is about as usual. I do not believe that my engine has vacuum leak, since I just replaced the lower/upper manifold gaskets and RTV sealed them properly, without any more oil/coolant leaks!!