Bang. The washing machine finishes filling and something inside the wall knocks hard enough to rattle a picture frame. Or somebody shuts the kitchen faucet quickly and there is a thud under the floor.

That noise is called water hammer, and it is more than an annoyance. Each bang is a pressure spike traveling through your pipes, fittings and appliance hoses. Once you know what sets it off, most of the fixes are small.

A column of water that stops all at once

Water moving through a pipe has weight and momentum. When a valve closes slowly, the water eases to a stop. When a valve snaps shut, the moving water slams into it and a shock wave runs back down the line. The pipe jumps, and if it has any room to move, it hits a stud or a floor joist. That is the bang you hear.

The worst offenders are valves that close electrically in an instant: the fill valves in washing machines, dishwashers and some ice makers. A single-handle faucet slapped closed can do it too, usually more quietly.

Notice which fixture sets it off. A noise that follows only the washer has its fix near the washer. If every faucet in the house makes it, the cause runs through the whole system.

Timing helps too. A washer fills several times per load, so a bang that repeats every few minutes during laundry is almost certainly its fill valve. A single thud a second or two after a faucet closes is often the pipe settling back after the shock, and a rattle that keeps going for a moment tells you the pipe is loose enough to bounce.

Loose straps in older walls

Pipes are held to the framing with straps and clips. In homes that have been lived in for decades, those straps loosen, corrode or were spaced far apart to begin with. A pipe that is not held tight has room to swing, and a shock that would be silent in a well-strapped wall turns into a loud knock.

Where the pipe can be reached, under a raised house or in an unfinished space, securing it with proper straps and cushioned clips can make a big difference. Behind finished walls it takes more work to get to, which is one reason we track down where the noise starts before we open anything.

A loose pipe also wears where it hits. A copper line that has been tapping the same stud for years can thin out right at that spot.

Is your house pressure too high?

High pressure makes every shock worse. The same fast-closing valve that makes a soft thud at moderate pressure can crack like a hammer blow when the pressure is high. Many homes have a pressure regulator on the main line, a bell-shaped valve near where the water comes in, that holds pressure down to a safe range. Regulators wear out, and a failing one often lets the pressure creep up.

A gauge threaded onto a hose connection tells you what the house is running at. If it reads high, adjusting or replacing the regulator comes first. It usually quiets the hammer, and it also eases the wear on faucet cartridges, toilet fill valves and the relief valve on your water heater.

Faucets dripping more than they used to, toilets that keep refilling and appliance hoses that fail early are all reasons to check the pressure, bang or no bang.

What an arrestor does

A water hammer arrestor is a short sealed chamber with an air cushion or a piston inside, fitted on the supply line close to the valve causing the trouble. When that valve slams shut, the shock pushes into the cushion instead of rattling the pipe.

The most common spot is the washer box, one on the hot line and one on the cold. Dishwasher supplies come next. Some older homes have homemade air chambers, a plain capped stub of pipe standing up near a fixture. Over the years those fill with water and stop cushioning anything, and draining the house lines can bring them back for a while.

On an open washer box, arrestors go in quickly. When the noise comes from deep inside a wall, we deal with the pressure and the straps first.

When the noise says a line needs attention

A bang that shows up suddenly in a house that was always quiet is worth a look. So is one that gets louder over a few months, or one that arrives with a new drip under a sink, a damp spot on a ceiling or a fitting weeping near the water heater. Repeated shocks work joints loose over time.

Check the hoses on the washer while you are at it. A rubber fill hose takes that same shock every time the valve snaps shut, and an old hose with a bulge or cracking near the fittings is one of the more common ways a laundry room ends up flooded. Braided hoses are a small upgrade worth making.

In established Pearl City homes still running on their original water lines, water hammer can be one more sign those pipes are near the end. If we are chasing noise, loose straps and fresh leaks in the same old runs, we will tell you plainly whether patching them one at a time still makes sense. Our Pearl City repiping page covers the bigger fix, the repiping page explains PEX and copper, and a plumbing inspection gets you an honest read on the whole system first. Photos from past repipes are in our gallery.

Nobody should flinch every time the dishwasher finishes. A strap, a regulator or a pair of arrestors quiets most banging pipes, and the ones that stay loud are telling you something about the pipe.