Kilik is right on this one. Not only is it vibration, but even "vibrating crystals"

that regulate a processor speed. Quartz crystals are piezoelectric, which means, as Kilik said, that changing their shape produces voltage, and applying voltage changes their shape. So electricity can make them vibrate, which is useful for things like watches. The CPU uses quartz crystals the same way a watch does. The vibration from the crystal regulates the "clock speed" of the CPU. This makes sure that the operations of the CPU don't get ahead of themselves and cause it to overheat.