Fixing Rattles/Hums from Ultra 80...

I've got an Ultra 80 that's got a low pitched, oscillatory hum. Best way I can think to describe it in text is like a "huuumm....... huuumm....... huuumm....... huuumm....... " that occurs at even intervals, about every 1.5 seconds or so. It used to be accompanied by a louder metallic rattle, but I fixed that by gluing some foam to front bezel--between where it locks to the bottom of the front metal panel and where it just touches it closer to the screws. But the low-pitched hum is easily as annoying, and I can't trace it.

Part of the problem is that the machine won't operate without the side panel in place; there's a switch that automatically cuts the power when that is missing. Thus, I can't search inside the case while it's running to pinpoint the noise.

On a related note(?), I noticed that the system always operates at the same volume, regardless of load: the sound it makes when it boots up is the same sound it makes if it's rendering a 3D scene, for example. Does that mean the fans are always running at max speed?