Quick Answer:
If you hear a single knock or jolt under the sink right when the dishwasher finishes its fill, it’s usually a pressure shock from the dishwasher’s inlet valve stopping water suddenly. Start a cycle and wait until the initial fill ends, then place your hand on the dishwasher supply line to feel the jolt. Confirm the dishwasher stop valve is fully open, then close it about 1/8 turn and retest. If the shutoff valve is corroded or starts to weep after you touch it, stop and call a plumber — they can replace the valve without damaging the cabinet.
Why This Happens
Modern dishwashers use a solenoid valve that opens to let water in and snaps closed when the fill is finished. That quick shutoff creates a pressure spike that travels along the metal supply line and can cause a sharp knock where the line or fittings meet the valve or nearby pipes. A partly closed or stiff stop valve makes that jolt worse because it changes flow and pressure dynamics at the instant the valve closes.
Other common causes include loose pipe straps or nearby plumbing hitting the cabinet, but the loud single knock tied to the end of the fill is most often a fast pressure change, not a loose fixture. Don’t confuse pump or drain vibration with a hammer-type knock — see Banging noise when hose turned off if the sound happens when you shut other fittings, or Banging started after plumbing repair if the problem began after recent work.
Step-by-Step What to Do
Start a cycle and wait for the initial fill to end
- Run the dishwasher so it begins a normal fill. Listen for the water flowing into the machine and wait for it to stop — that’s when the solenoid has closed.
- Keep the cabinet doors open so you can work and observe safely.
Place a hand on the supply line to feel the jolt
- Reach under the sink and place a palm on the dishwasher water supply line or the metal valve body directly above it.
- When the fill ends, you should be able to feel a small jolt or thump in the pipe. That confirms the noise is coming from the incoming water side, not the drain pump.
Confirm the dishwasher stop valve is fully open
- Locate the small shutoff (angle) valve on the branch feeding the dishwasher. Turn it fully counterclockwise until it stops. If it’s a quarter-turn ball valve, make sure the handle is parallel with the pipe.
- Do not force a valve that feels seized or brittle; forcing a corroded valve can break it and cause a leak.
Close it 1/8 turn and retest
- From fully open, close the valve a very small amount — about an eighth of a turn — to slightly reduce flow and soften the pressure change.
- Run the dishwasher through another short test fill. If the knock softens, the valve or flow condition is contributing to the shock. If nothing changes, or the valve leaks after you touch it, stop here and call a plumber.
- If the small closure helps, plan to have the valve inspected and replaced by a pro for a permanent fix rather than leaving it partly closed long-term.
What Not to Do
- Don’t confuse drain-pump vibration for the hammer-type knock — the drain pump makes continuous vibration or rhythmic noise, not a single sharp jolt at the end of a fill.
- Don’t crank down compression nuts or try to overtighten fittings as a first fix; that can damage fittings and cause leaks.
- If the stop valve starts to seep after you touch it, stop and have a plumber replace the valve rather than attempting home repairs on a corroded valve.
When to Call a Professional
- Call a plumber if the valve is corroded, stiff, or begins to leak when you move it — a pro can replace the shutoff valve without damaging the cabinet or surrounding plumbing.
- Call if the knocking continues after the steps above, if pipes are loose enough to hit the cabinet, or if you’re unsure how to safely isolate and test the supply.
- If there’s any active leaking, shut off the house water and call a plumber right away.
Safety Notes
- Turn the dishwasher off between tests so it doesn’t advance unexpectedly while you’re working in the cabinet.
- Have a towel and a small bucket handy in case a fitting starts to drip during testing.
- Wear gloves and eye protection if you work under the sink. Don’t force stuck or corroded valves — that can cause sudden damage and a larger leak.
Common Homeowner Questions
- Q: Why does the knock only happen when the fill stops?
A: The solenoid closes quickly at the end of the fill, creating a pressure spike that causes the knock. - Q: Will replacing the valve damage the cabinet?
A: No. A qualified plumber can replace the shutoff valve without cutting or damaging typical cabinets. - Q: Could tightening loose pipes solve it?
A: If pipes are rattling, securing them helps, but start with the valve checks — the pressure spike is the most common cause.
For more related articles, see the Pipes Banging After Fixtures Turn Off hub.