I just got a 2.66 Quad-core Mac Pro. I've read a lot of comments about how quiet they are, things like "silent", "can't tell that it's running", etc. Mine is certainly quieter than my G5, but it is far from silent. I hear the typical high-pitched whine that I expect from computers, not loud, but there. But the sound that's starting to bug me is a low-frequency sound, almost like motor. It oscillates in volume from. It's not loud, but it has an irritating quality. I can't tell where it emanates from almost silent to too loud. It's not the graphics card or the disk drive, and all the fans are running under 1000 rpm. I'm guessing this isn't normal, but does anyone have an idea of where the sound is coming from? Thanks.