Likely cause: water hammer at one fixture

Why this happens

Family Handyman says water hammer comes from the quick shutoff of a supply line: the energy in the flowing water has to go somewhere, so pipes flex and hammer against nearby studs, joists or other pipes. It says valves triggered by a solenoid, like the fill valves in washing machines, dishwashers and water softeners, shut off almost instantly, while hand-controlled faucets usually cause less hammering. Bob Vila says to take it seriously, because it can damage pipe connections over time.

How to fix it

1. If you smell gas at any time, leave the house and call 911 and your gas utility from outside. The Pennsylvania PUC says not to turn lights on or off or make calls from inside.
2. Find the source. Family Handyman says washing machines and dishwashers are prime suspects because their automatic valves close fast.
3. Look at pipes you can see in a basement, crawl space or utility room. If one moves or knocks on framing, tighten the strap with a screwdriver or add a strap, as Bob Vila describes. Bob Vila says never to use steel or galvanized straps on copper pipe.
4. Ask a plumber about a water hammer arrester. Family Handyman describes one as a piston or diaphragm between the water line and an air-filled chamber that absorbs the surge. Sioux Chief says arresters go on the hot and cold supply lines for washing machines, dishwashers, ice makers and quick-closing tub or shower valves, within six feet of the valve that causes the bang.
5. If only the washing machine bangs and you are comfortable with it, Family Handyman says some arresters mount between the faucet and the washing machine hoses. Water off comes first. Close both the hot and the cold washer valves by hand, and never force one. Put a towel and bucket down, away from the washer's plug and the outlet, then disconnect the hoses, as Oatey and Bob Vila describe. Bob Vila and Sioux Chief say to fit one arrester on the hot line and one on the cold line. Follow the arrester's own instructions, reconnect the hoses, turn the washer valves back on by hand, and check every connection for drips. Oatey says to restore the water supply, and Bob Vila says to check the connections for leaks. If a valve will not turn or a fitting drips, close the valves, stop and call a plumber.
6. If water spills near the washing machine's cord or an outlet, stay out of the water and do not touch the plug. Ready.gov says not to touch electrical equipment if it is wet or if you are standing in water.
7. If the fix would mean opening a wall, or cutting or soldering a pipe, leave it to a plumber. Family Handyman notes that some arrester installs require opening walls.

When to stop. If you see a drip, a damp spot or a stain at any pipe or fitting, shut off the main water valve by hand and call a plumber. Never force a stuck valve.

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