theres a bios setting on some motherboards which is sometimes called smart fan control. If it is enabled the fans only switch on when the processor reaches a certain temp and then switch off when it cools down. It sometimes seems to get stuck in a endless loop of it cooling down by a degree or so, switching the fans off then it heats up again fans go on then off and the cycle repeats. You switch it off, if it is in the bios settings, although the fans will be constantly.