That bang in the wall when the washing machine shuts off is not a ghost and it is not normal. It is water hammer, and it is slowly loosening every joint in your plumbing.

I lived with it for two years because I assumed it was a structural quirk of an old house. Then a plumber told me the fix is usually fifteen dollars and twenty minutes.

What is actually happening

Water moving through a pipe has momentum. When a valve closes fast, that moving column of water slams to a stop and the energy has to go somewhere. It goes into the pipe, which jumps and hits the nearest joist. That is the bang.

Newer houses have air chambers near the fixtures to absorb it. Those chambers fill with water over the years and stop working. Which is why old houses are the noisy ones.

The fix, cheapest first

1. Lower the water pressure. If your system runs above 80 psi, that alone causes hammer everywhere. A pressure gauge screws onto a hose bib and costs about ten dollars.
2. Add a water hammer arrestor. These are small capped cylinders you install between the shutoff valve and the supply line at the offending fixture. They have a sealed air pocket so they never waterlog.
3. Secure the loose pipe. If the bang is loudest in one wall, that pipe may simply be unclamped. Adding a plastic clamp or a strip of foam insulation can quiet it a lot.
4. Replace worn washer valves on older faucets. A valve that slams shut is half the problem.

Start with the arrestor at the washing machine. That is where hammer is loudest and most common, and the machine is usually easy to reach.

What not to bother with

• Draining the whole system to restore old air chambers works for about a month and then they waterlog again.
• Wrapping pipes in towels does nothing except hide the problem and make a wet mess.
• Overtightening clamps can crush copper tubing. Snug, not cranked.

The one thing worth checking before you buy anything: turn off every fixture in the house except the noisy one and listen. If the noise disappears, the culprit is pressure, not the fixture, and the arrestor will not help. That two minute test saved me from buying the wrong part.

I fixed ours with a fifteen dollar arrestor behind the washer and a clamp on one loose pipe in the basement ceiling. No bang for eight months now, which is long enough that I finally stopped flinching.