Very Computer

Hello.  Recently, the loudness of the noise generated by the fan of my ol'
4D/20G has been varying, alternating between intervals of louder and
softer volume levels, and I was wondering if this could possibly be a sign
of a problem that I should worry about.  The fan seems to still be
working, just judging from the fact that I can feel the air blowing out
when I put my hand behind the box.  So, perhaps this could simply be
caused by a fluctuating level of power the machine receives from the
outlet???  (I'm no expert on hardware so it's a complete wild guess, but
the ceiling light in the room that the computer is in does also sometimes
temporarily get a little dimmer.) I would appreciate hearing from anyone
who may have come across this sort of thing before, and/or have a better
guess as to what could be causing this to happen.  Thanks!!!!