what's up all!

when cruzing down the road at night, I notice my head lights amount of glow will vary relative to the RPM of the engine. higher RPM/brighter light...Lower RPM/dimmer light. Also the check engine light and one other light(I can't remember) on the dash will bairly glow. these don't come on, but sort of bairly shine the same way my headlights do relative to the engine RPMs. I only notice these at night though as I probably just can't see them during the day. Anyway, my question is...Is this a sign of a failing alternator? If not, any ideas on what it might be? I just replaced all the belts and have them super tight to prevent any squealing, which they don't do. So I don't think the belt is slipping or anything.....besides, I noticed the dash light thing before I replaced the belts. Thanks for any input.