Symptoms
- Loud thud or knock right when a faucet, washing machine, or dishwasher valve closes
- Noise seems to travel through the walls
- Happens consistently at the same point in a cycle
- Pipes may vibrate briefly
Likely causes
- CommonMissing or failed water hammer arrestorThese small shock-absorbing devices are often missing in older homes or wear out over time.
- CommonQuick-closing solenoid valves on appliancesWashing machines and dishwashers use fast electric valves that stop flow abruptly, which is the most common trigger.
- PossibleHigh water pressure throughout the houseExcess pressure makes the shock of stopping water more forceful.
- PossibleLoose pipes not properly secured to framingUnsecured pipes amplify the banging sound as they shift slightly on impact.
What to do
- Note which fixture or appliance triggers the banging to narrow down where to address it.
- Check your water pressure with a gauge on an outdoor spigot; above 80 psi contributes to hammer and may need a pressure-reducing valve.
- For washing machine or dishwasher lines, install a water hammer arrestor at the appliance's supply valve, which usually just screws on.
- For a whole-house issue, arrestors can be installed near the main lines by a plumber, or a whole-house pressure regulator can be added.
- Check accessible pipes (basement, crawlspace) for looseness and secure them with pipe straps or cushioned clamps.
- If your home has old air chambers instead of arrestors, try draining the system (shut off main, open all faucets) to restore trapped air temporarily.
- Retest after each change to confirm the banging has stopped or lessened.
Tools and parts
- Water pressure gauge
- Wrench
- Pipe straps
Water hammer arrestorSearch: “water hammer arrestor washing machine”
When to call a pro
Call a plumber for a whole-house pressure regulator install or if arrestors at individual fixtures don't resolve the noise.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.