While sitting idle, the fan on my mac pro (well, I think it's one of the fans) does this strange cycling thing. At first I thought it was a truck way off in the distance, starting and stopping. Then I realized it was my Mac Pro. Sitting with no apps running, one of the fans frequently cycles on an off audibly. I recorded it. Have a listen. You have to listen past the low frequency rumble of the Mac and you'll hear the fan spinning up and then spinning down in cycles that are typically 8 or 10 seconds, sometimes shorter. The sound file is hosted here.