One of the most common causes of humming pipes after flushing a toilet is debris trapped in an inlet valve. Water carries small stones, calcium deposits and solder to the valve area which traps them, resulting in the annoying noise.
Before replacing the valve, owners might follow the manufacturer's directions to flush any debris from it. Fluidmaster says to turn off the water supply, hold onto the valve shaft with one hand and flip the top cap off with the other. An inverted cup over the open valve prevents water spills while turning the supply valve on and off several times to eliminate the debris. After flushing the valve, it is ready to reassemble.
If homeowners are able to feel the vibration responsible for the hum, installing insulation between the pipe and the wall often muffles the sound. Plumbing supply houses carry affordable foam pipe insulation that is ideal for this purpose.
Humming pipes are sometimes an indicator that water pressure is too high. In homes where the water supply is from a well, the homeowner has the option of lowering pressure at the pump and pressure tank. In homes using a municipal supply, owners should consider contacting a plumber to install a pressure reducing valve.