One of the most common causes of plumbing hums is a problem with water pressure. If the pressure is too high, the variations that occur when opening and closing a valve result in both vibration and hum. A faulty toilet valve is another common cause of the noise.
If the water supply to a home comes from an on-site well, the homeowner is often able to solve the problem by making a pressure adjustment at the pressure tank. If the home is on a municipal supply, having a plumber install a pressure-reducing valve in the line near the shutoff valve gives the homeowner better control of the pressure and allows him to eliminate the humming pipe. The normal setting is 55 pounds per square inch or lower. Avoid setting the pressure too low to maintain a good flow volume.
If the noise only occurs near a single toilet, the valve that controls the water to fill the tank is sometimes at fault. While the hum is still due to a pressure problem, older toilets used metal valves that magnify the vibrations to an audible level. Replacing the older valve with a newer plastic one often eliminates the noise. Replacing the valve is a task that is within the abilities of many homeowners and is a more affordable fix than installing pressure-reducing valves.