If the hum is moderate, that is, not at full volume, then it is probably in the power supply. Often a large filter capacitor will break away from the circuit board or go bad. If the hum is extremely loud, what I would call "scary loud" so that you are compelled to pull the plug immediately, then it could be a shorted diode or output device. Anyway, it's not the switch, but it still could be easy to fix.