Thumping in the water pipes is known as water hammer, and it happens when water, which is incompressible, changes direction suddenly. You're most likely to hear banging pipes when the water is turned off, and it can happen near water-using appliances, such as washing machines and toilets, when the automatic fill valve shuts off. The water has to stop suddenly, and since it doesn't have any place to go, it pounds against the walls of the pipe.
Water hammer can also happen when you turn the water on, and sometimes you'll notice it when a particular faucet, such as the one in the shower, is partially on. A certain flow rate in the faucet creates a resonant vibration that causes the water to bounce back and forth in the pipes. A cold or hot water tap making a loud noise is almost certainly a case of water hammer.
The vibrations caused by water hammer aren't good, and it's important to do something about the problem as soon as possible. Besides being annoying, the vibrations and banging can cause leaks. You may be able to fix the problem by emptying and refilling your water pipes, but if the problem happens only at a single fixture, a better idea is to install a water hammer arrestor near that fixture.
Vertical Air Chambers Are Supposed to Prevent Water Hammer
Many residential plumbing systems are designed with vertical air chambers placed strategically throughout the pipe network. The idea is that the air in these pipes is compressible and can absorb the force of the water as it changes direction. The problem with this design is that the air chambers often fill with water, which renders them ineffective.
As a first step in fixing a water hammer problem, plumbers recommend draining your water system completely to get the water out of these air chambers. When you refill the pipes, and water seals the air in these chambers, the banging should stop. Unfortunately, they'll probably fill with water again, so you'll have to repeat this procedure periodically.
A Kitchen Faucet Makes a Thump Noise
When water hammer affects a single fixture, such as a kitchen or bathroom faucet, you may be able to stop the banging by servicing the faucet and replacing the valve. The existing valve may be loose, forcing water to stop and start when you turn on the faucet. A kitchen faucet diverter thump that occurs when you turn on the sprayer can also sometimes be corrected by servicing the faucet.
If the faucet is new, or servicing it has no effect, the next step is to install a water hammer arrestor in the pipes just before the shutoff valves. You can also stop thumping in the bathroom sink, shower or toilet by installing a water hammer arrestor, but you need access to the pipes just before the valve, and that may mean cutting into the wall.
What Is a Water Hammer Arrestor and How Does It Help?
A water hammer arrestor is a plumbing fitting that consists of a closed cylindrical chamber about 3 to 4 inches long attached to a tee. Inside the air-filled chamber, a spring-loaded piston moves back and forth, absorbing the excess pressure in the pipes that causes the banging.
You install a water hammer arrestor by cutting into the pipe on the supply side of the washing machine, faucet or toilet shutoff valve as close to the valve as possible. The tee can either be soldered onto the pipes or installed with compression fitting, depending on the style. Although you may have to remove part of the wall, installing a water hammer arrestor is an easy job for a plumber or a home DIYer with plumbing experience.