Symptoms
- Loud knock or thud right as the washing machine finishes filling
- Sound repeats at each fill cycle
- Noise seems to come from the wall behind the machine
- Other fixtures don't cause the same noise
Likely causes
- CommonFast-closing solenoid valve in the washing machineThese electric valves shut almost instantly, creating a pressure spike that older plumbing wasn't designed to absorb.
- CommonMissing water hammer arrestor on the washer's supply linesModern codes often require these, but many existing setups lack them.
- PossibleHigh water pressure amplifying the shockExcess house pressure makes the sudden valve closure more forceful and the bang louder.
- Less commonLoose supply pipes behind the wallUnsecured pipes can amplify the sound even with an arrestor installed.
What to do
- Confirm the noise happens specifically at the end of the fill cycle, not throughout the wash.
- Turn off both hot and cold water valves behind the washing machine.
- Disconnect the supply hoses and install washing machine-specific water hammer arrestors between the valve and the hose on both hot and cold lines.
- Reconnect the hoses to the arrestors and turn the water valves back on.
- Run a test cycle (or just fill briefly) to check whether the banging has stopped.
- If it persists, check your home's overall water pressure with a gauge, since high pressure may need a separate pressure-reducing valve.
- If pipes behind the wall are known to be loose from prior work, that may need addressing separately by a plumber.
Tools and parts
- Wrench
Washing machine water hammer arrestorSearch: “washing machine hammer arrestor”
When to call a pro
Call a plumber if arrestors don't resolve the noise or if you suspect loose pipes within the wall.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.