It is not uncommon in most homes to have a variety of noises coming from plumbing pipes and fixtures. Moaning in your home's plumbing can sometimes be attributed to a faulty ballcock, or fill valve, in one or more toilets. Water pressure that is set too high can also cause moaning noises. Both problems are inexpensive and fairly simple to fix as long as you properly diagnose them.
Check that your water pressure is set to no more than 80 psi, or pounds per square inch. Measure water pressure using a psi gauge with a hose adapter. Turn off all the faucets in the home prior to testing to get an accurate static reading. This includes washing machines, dishwashers and anything else that draws water. Attach your gauge to an outlet such as that for the washing machine. Once the gauge is in place, turn on water to that one outlet, and read the psi. Turn the water off and disconnect your gauge.
Most home water supplies are set at 40 to 60 psi. If your pressure reads higher than that, install a pressure regulator to protect your plumbing from water pressure damage and to reduce plumbing noises. If your pressure reads above 70 psi, use the regulator to set it to the recommended 60 psi, and see if the moaning stops.
If the fill valve on your toilet is faulty, pipes may make loud moaning noises. Other noises to indicate this problem are a foghorn sound, wailing, humming or oboe-type sounds. The fill valve is located at the bottom of the tank. In most toilet models, a rod extends upward and supports the float arm and attached float ball.
Testing Fill Valve
Test the fill valve by shutting off water to all toilets in the house and opening them back one at a time until the noise starts again. Replace the fill valve assembly in that toilet. You may also test by lifting each ballcock out of the water while someone else starts a faucet. Choose the faucet that most often causes the the noise when it is running. If the moaning stops, replace the fill valve assembly in that toilet. This testing method is ideal if the shutoff valves for the toilets are difficult to access.