Hi! The low humming sound is probably from your low voltage power still on. The hum could probably be the contactor or reversing valve on a heat pump being energized. The low voltage power transformer is usually located on the indoor furnace or air handler. I would suggest turning the power off to the furnace or air handler. Usually there is a switch on the side of the furnace to do this. On an air handler there are usually breakers or a disconnect. The low voltage will not hurt you, but if you accidentally touch a low voltage wire and ground it out then you might damage your board or blow a low voltage fuse on the board. Best of luck on finding your problem.