The sound you are describing is usually caused by a toilet fill valve that is slowly leaking. To locate the leaking toilet, remove the lid of each toilet tank and adjust the fill valve mechanism until it stops. Once you have found the noisy toilet, repair or replace the fill valve.
This is typically the toilet's fill valve. When the diaphragm gasket inside the top cap of the fill valve goes bad, it loses elasticity and becomes hard. When it tries to shut off, it bounces in micro vibrations sending a shock wave back into the piping. You can test the fill valve theory by removing the tank lid and lifting gently on the fill valve float arm. If the noise stops, you've found your problem and should proceed with replacing the fill valve.
Improperly secured pipes behind walls may be the cause of the plumbing noises you’ve described. Since water supply pipes are made of metal, they transmit sound waves. As water flows through the system, it flows over obstructions and around turns, creating eddies in the water. These can be rhythmic depending on the speed of the water. When they reach a certain pitch, they cause the piping to begin vibrating in harmonic reaction to the wave created inside the pipe. In order to quiet your noisy water pipes, you’ll have to remove the offending part or debris that's causing the water to fluctuate. If the pipes are accessible, you can strap them with plastic clamps so they can't bang against the structure. If your piping is metal and runs through the joists or studs, then the problem may be expansion and contraction. As soon as either hot or cold water runs through pipes, they expand or contract, causing vibration and noise.
Another problem is water hammer. This happens when you have high water pressure. Water moving in one direction does not want to stop flowing. When you shut off a faucet, the water still has some force, which has to be absorbed, causing the pipe will flex. If the pipe is near wood, it will bang against the wood. You can stop this plumbing noise problem by installing water hammer arrestors on the offending water lines.
Rumbling sounds coming from a water heater are an indication that sediment has built up on the bottom of the water heater. What you are hearing is water that is trapped in the sediment and is boiling. This is an indication that the water heater is not operating efficiently. Sediment will not allow the heat to transfer to the water in the tank, which sends the heat up the flue.
You may try draining a few gallons of water off the bottom of the water heater tank. This is done by attaching a drain hose to the valve at the bottom of the tank. Allow it to drain for about five minutes.
WARNING: HOT WATER IS DANGEROUS. DISCHARGE THE WATER INTO A FLOOR DRAIN, LAUNDRY TUB OR BATHTUB. HOT WATER WILL KILL YOUR GRASS IF DISCHARGED ONTO THE LAWN. HOT WATER WILL CRACK A TOILET BOWL IF DISCHARGED INTO THE TOILET.
Many newer models of water heaters have a new feature that prohibits the build-up of sediment in the tank. If your heater is an older model, it may be cost effective to replace the water heater if the build-up is severe.
These noises often occur when the pipes are beginning to develop build-up on the inside. Often galvanized pipe and copper pipe will have these types of noises, because of the harder turns and the reverberation that occurs because the pipe is metal. Galvanized pipe often develops calcium and mineral deposits on the inside, which causes the water to swirl and create noisy water pipes.