I recently purchased a ReadyNAS NVX, a 4-drive NAS storage device. It is a nice, solidly built, small package, and generally is not too loud. Most other sounds drown it out, so I have it sitting out in the open in my living room next to a Media Center PC that drives my TV.
Every so often, too frequently for me to just ignore, the fan on the NVX jumps up a notch (to around 2200-2500 rpm) and gets much louder. Usually within 10 minutes, it jumps up another notch (to around 3200 rpm), and is really loud. Loud enough that it interferes with watching any kind of media.
I am at a loss to explain why this happens. The temperature of the room is pretty stable, on a scheduled timer and temperature. When I am at home and watching something, the temperature is always about 70-73 degrees. Given the consistency of the temperature, I can not figure out why the NVX's fans randomly spin up. The drive temperatures are consistently at 44-45 degrees Celsius, and when the fans spin up there seems to be no corresponding change in the temperatures of the NVX or the drives.