Water hammer (or hydrostatic / hydraulic shock) causes plumbing supply pipes to make a noisy banging sound when valves are turned off quickly. It can be quite loud, annoying…and ultimately cause the pipe to fail.
It occurs because there is momentum and pressure built up in the water lines. When the water supply to any fixture in the home is shut off, for example, a toilet, dishwasher, etc. it may occur. It may present itself as one loud bang, repeated banging or simply the rattling of the pipe. Over time this can damage the pipes / pipe connections and cause them to leak. A vacuuming effect can also be created when water downstream attempts to continue flowing and may cause the pipe to collapse or implode.
Poorly supported pipes, certain plumbing fixtures or longer runs of pipes can contribute to the problem. Installing an arrester or noise suppression device closest to the faucet or fixture making the noise often fixes the problem.
If you ‘re not sure what all that banging noise is when you turn off the water, chances are it’s water hammer.