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.

by in Moorpark, CA on August 01, 2013
1 answer 1 comment
ANSWER by , August 03, 2013
Does the humming come and go as well and at the same time as the Idle changes? How much RPM variation is there?
COMMENT by , August 05, 2013
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!!