1995 Chevrolet Lumina Q&A

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

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. -
Answer 1
Does the humming come and go as well and at the same time as the Idle changes? How much RPM variation is there? -
Comment 1
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!! -