The short answer
A rhythmic knock is nearly always a spray arm striking a tall item, and it stops when you reload. A grind or rattle during the drain usually means something hard has reached the pump, and a hum that has grown louder over time is the wash pump or the machine touching the cabinetry.
What the noise does
- A regular knock during the wash — A spray arm hitting a dish or a handle.
- A grind in short bursts near the end of the cycle — The drain pump with something in it.
- A loud hum throughout the wash — Circulation pump or resonance with the cabinet.
- A rattle that changes if you press on the door — The door panel or the machine touching the carcass.
- Squealing at the start of a cycle — A pump starting dry or a bearing complaining.
- Not this fault: silence where a wash should be — That belongs on the cycle page.
Ranked for built-in machines
- Spray arm striking an item in the rack — First, and it needs only a reload to prove.
- Foreign object in the sump or the drain pump — Second: fruit stones, glass and labels are the usual finds.
- Machine not level or touching the cabinet sides — Third, and it makes ordinary noise sound like a fault.
- Spray arm bearing worn so the arm wobbles — Fourth, and it usually hurts cleaning too.
- Wash pump bearing worn — Fifth, and it presents as a hum that has grown.
- Drain pump impeller damaged — Last, and it follows a foreign object that was never removed.
Diagnose in this order
- Open the machine and spin each spray arm by hand — Anything it strikes will be obvious.
- Reload with tall items away from the arms and rerun — It confirms or clears the leading cause.
- Listen for whether the noise is constant or in bursts — Wash and drain sound different and happen at different times.
- Check the machine is level and not touching the cabinet — Free, and a common finding.
Owner checks
- Turn both spray arms by hand through a full rotation — Anything in the way makes itself known immediately.
About 5 min
- Remove the filter and look into the sump for hard debris — Wear gloves; glass fragments are the common find.
Tools: gloves, torchAbout 15 min
- Reload the racks so nothing overhangs into the arms — Then run the same program again as a test.
About 20 min
- Check the machine sits level and its sides do not touch the cabinet — Adjust the feet if it rocks.
Tools: spirit level, adjustable wrenchAbout 20 min
- Note exactly when in the cycle the noise happens — It saves a technician a whole visit.
About 10 min
- Stop before removing a pump or dismantling a spray arm mount — Those are under the tub among mains connections.
Before reaching inside
Safety
Unplug the appliance or switch off its breaker before removing any panel. A capacitor can hold a charge after the power is off. Let the machine cool after a hot cycle before reaching into the tub, wear gloves in the sump because broken glass collects there, and unload the cutlery basket before working inside.
When to call
Call when the racks are reloaded, the sump is clear and the noise is still there, because the remaining causes are pump bearings and arm mounts under the tub. Call promptly for a grinding drain: something hard is passing through a pump that was not designed for it, and the damage accumulates with every cycle.
Other faults on this machine
Scope of this page
Generalized across dishwasher families under this badge; where a family-specific or model-specific page exists it is the narrower answer. Not here: pump part numbers, spray arm assemblies, bearing specifications and the installation clearances that differ by kitchen.
Lead Appliance Repair Technician · 12 years experience
Experienced technician with 12 years specializing in washing machine and dryer repairs across all major brands.
