You normally will not hear noises from your house's plumbing, so a humming noise will catch your attention as a potential problem. Multiple factors will cause the plumbing in a house to hum, but determining the cause takes time and patience. In the end, an expert may be the only way to find the cause.
Shut Down Valves
A water valve that is wearing out may cause humming in the pipes, giving you an early indication of a problem before you are dealing with a flood. Go around your house and close the different water valves one at a time. Listen for the humming noise before you close the valve, and listen for it after you close the valve. If the humming noise stops after you close the valve but restarts after you open the valve, you need to replace the valve.
Leaks may cause humming as the pipes experience a loss in water pressure from the leak. For the humming to be loud enough you can hear it, the leak must be noticeable. Check each faucet in your house for leaks, as well as the valves under the toilets and sinks. Do not forget to check the outside faucets. Notice any water damage on walls or ceilings as well, indicating a leak behind walls or in subfloors.
Sometimes the pipes in your house will contact an appliance that vibrates, leading to humming in the plumbing. A pipe can sit under the floor where your fridge is located. When the fridge's motor kicks on, the floor vibrates along with the pipes. An air-conditioning unit may touch pipes, vibrating them each time the air conditioner's fan kicks on. Turn off different appliances for extended periods of time to eliminate them as the cause of the humming.
Draining the System
Draining your house's plumbing may eliminate the humming if you still cannot find the cause. Close the main water valve in the house, and then turn on a faucet or two to drain out any leftover water. Turn off the faucets once the flow of water stops, and then open the house's main water valve. Turn on each of the faucets in your house until you see a constant flow of water come out, and then flush all of the toilets. If the humming continues, contact a plumber for help.