The short answer
Grinding, rattling and knocking from a dishwasher have short, checkable causes: debris in the sump, an item touching a spray arm, or the drain pump working on something it cannot pass. On a B-Flasche dishwasher the order below is what makes this page worth reading — the causes are ranked for this kind of machine, not listed alphabetically.
The symptoms, and the near misses
- A grinding or crunching noise during the wash — Hard debris in the sump being drawn through the pump — glass, a seed, a fruit stone.
- A rhythmic knock as the spray arm turns — The arm is striking an item in the rack. It stops the arm and leaves half the load dirty.
- A buzz or hum at the drain step — The drain pump loaded up or jammed, which is a different sound from the wash pump.
- Rattling from the rack itself — Items touching each other. It is noise without a fault.
- NEAR MISS — a hum with no water movement at all — That is a pump that is not pumping, which is a drainage or circulation fault rather than a noise complaint.
What it usually turns out to be
- Debris in the sump or the pump inlet — First. The filter catches most of it, but glass chips, seeds and stones get past and grind against the impeller every rotation. It is the single most common cause and it is reachable by hand.
- Spray arm striking the load — Second, and free to fix: a handle hanging through the basket or a tall item in the centre stops the arm dead and produces a knock every time it tries.
- Worn wash-pump bearing — Third. A continuous drone that gets worse over months, unaffected by loading, and it does not clear when the sump is emptied.
- Installation — machine touching a cabinet or a pipe — Fourth: a machine not sitting square transmits its ordinary running noise into the carcass, which amplifies it dramatically.
The diagnostic order
- Spin both spray arms by hand after loading — If they catch on anything, that is your knock. It is a five-second check and it is very often the answer.
- Lift the filter and feel in the sump — Glass chips and stones are found by touch. Wear a glove and go carefully.
- Run the machine empty and listen — Noise with an empty machine is mechanical; noise only when loaded is the load.
What you can do yourself
- Remove the filter and clear the sump by hand — Use a glove — broken glass is the commonest hard debris in a dishwasher sump.
Tools: gloves, flashlightAbout 20 min
- Check the spray arms turn freely with the racks loaded — Spin each arm a full turn after loading. Move whatever it hits.
About 10 min
- Run an empty cycle and note when the noise occurs — Wash noise and drain noise come from different pumps, and knowing which halves the diagnosis.
About 60 min
- Check the machine sits level and clear of the cabinet — Adjust the feet so it stands square without touching the carcass or the waste pipe.
Tools: adjustable wrench, spirit levelAbout 25 min
- STOP before removing a pump — Both pumps are mains-wired inside a machine that holds water.
Working on this safely
Safety
Unplug the machine or switch off its breaker before removing a panel, and close the water supply before touching a hose or a fitting. Components inside can hold a charge after shutdown. Never run the machine with a panel off or with the lock defeated — the interlock is a safety device, not an inconvenience.
When to book a visit
Call when the sump is clear, the arms turn freely, the machine is level and a drone or grind persists through an empty cycle — that is a pump bearing or an impeller, and it means the machine comes out. Call sooner if the noise appeared suddenly along with poor cleaning: an impeller damaged by glass loses performance as well as making noise, and running it that way wears the housing.
Nearest pages of this class
Generalized information
A note on scope. This page generalizes across B-Flasche dishwasher families rather than naming one, so nothing on it depends on a model number and nothing on it should be read as a service-manual figure. The family- and model-specific pages beneath this one carry that level of detail; they are linked here, and if one of them names your unit it is the more specific — and more useful — answer.
Lead Appliance Repair Technician · 12 years experience
Experienced technician with 12 years specializing in washing machine and dryer repairs across all major brands.
