Dishwasher Knocking Noise: Causes & When It’s Serious

Quick Answer

A knocking noise during the wash is most often the spray arm striking dishes, tall utensils, or a misloaded rack. It happens because the arm spins fast and has limited clearance; one protruding item turns the arm into a “hammer.” First action: open the door mid-wash after you hear the knock, check for wet contact marks, and spin the spray arms by hand to confirm what they hit.

Identify the Noise First

During water fill: Spray arms are not the focus here. If the knock starts before any spraying sound, wait until the wash phase to evaluate arm contact.

During spray / circulation: This is the key timing. A spray-arm strike knock usually begins 30–120 seconds after fill, once you hear steady spraying/“whoosh.” It’s typically rhythmic and repeats every rotation.

During draining: If the noise happens only when water is pumping out at the end of a phase, it’s not a spray arm strike. Don’t chase rack/loading causes if the knock only appears during drain-outs.

During heating: Heating is usually quiet. If the knock continues only while the unit sits and heats with no spray sound, it’s not arm contact.

At start of cycle: A brief clunk as the unit starts spraying can still be a spray arm hitting a tall item the moment it begins spinning. Note whether it settles down after you rearrange the load.

At end of cycle: A knock right at the end is rarely spray-arm related unless the machine runs a final short spray. Confirm you hear spraying at the same time.

Constant: A true “constant” knock that doesn’t match spray bursts is unlikely to be an arm striking dishes. Spray arm knocks come and go with spray action.

Intermittent: Intermittent knocking that appears when cookware shifts, or only on certain loads, strongly matches spray arm contact. To isolate the stage, listen for the spray sound: knock + spray together points to arm strike.

What This Noise Usually Means

The spray arm is driven by water jets and rotates close to the racks and dishes. If a plate edge sticks out, a utensil stands upright, or a rack is slightly out of position, the arm clips it each pass. That impact transmits through the tub as a sharp knock or clack. Because rotation speed and load movement vary through the cycle, the knock may change cadence as the arm speeds up, slows down, or reverses direction on some designs.

Most Common Causes

Oversized/tall items protruding into the spray path: Spatulas, long-handled utensils, cutting boards, and tall cups can hang into the arc of the lower or upper spray arm.
Lower rack not fully seated on its track/stops: If the rack is one notch out or sitting on a dish, clearance drops and the lower arm hits the rack or items in the front corners.
Upper rack set too low for the load: Many racks have height adjustment. A low upper rack can put bowls or wine glasses into the upper arm’s sweep.
Dish placement extending past tines: Plates rotated so corners or rims cross the rack boundary get clipped as the arm passes.
Spray arm loose on its mount: A retaining nut/cap or center hub that isn’t secure lets the arm wobble, widening its swing and making it more likely to strike.
Spray arm warped or cracked: A heat-warped arm can “dip” and contact the rack/dishes even with a correct load.

How to Check Safely

1) Confirm cycle stage

Start a normal cycle and wait until you clearly hear active spraying. If the knock only occurs with spraying, proceed with spray-arm strike checks.

2) Isolate the load factor

Pause by opening the door when you hear the knock (let the spray stop). Without moving racks yet, look for the most likely interference points: tall utensils in the silverware basket, a pan handle sticking out, a cutting board leaning inward, or a cup flipped oddly.

3) Inspect relevant components

With the door open, spin the lower spray arm by hand. It should rotate freely without contacting dishes, rack tines, or the tub. Repeat for the upper spray arm (and mid-level arm if equipped). Any touch point is your knock source.

4) Confirm or rule out obstruction

Check for a dish that shifted and is now resting against the arm. Also verify the spray arm sits flat on its mount and doesn’t rock up and down. If it wobbles, don’t force it; note the looseness for repair.

5) Perform a controlled test

Run a short wash with the dishwasher empty except for the racks. If the knock is gone, it was load interference. If it persists empty, re-check each spray arm for looseness or warping by spinning and gently lifting at the tip; a properly mounted arm should not have excessive play.

When This Is Normal vs A Problem

Normal: A single light tap right as spraying begins that disappears after you re-seat a utensil or push the rack fully in. A knock that occurs only on overloaded or oddly shaped loads and stops with correct loading.

Problem: Knocking on every cycle regardless of load, knocking with the unit empty, or a heavy repeated clack that coincides with visible spray arm wobble. Also a problem if dishes come out chipped or if you see fresh scrape marks on the spray arm, rack coating, or tub surface near the arm’s sweep.

When to Call a Technician

Call for service if the knock continues with the dishwasher empty, if a spray arm is loose but won’t tighten securely, or if the arm is warped/cracked and replacement requires disassembly beyond a basic twist-off. Also call if the spray arm contacts the tub or racks despite correct rack seating and height adjustment, which can indicate a mounting or rack alignment issue that needs precise fitting.

How to Prevent This Problem

Do a “spin check” before starting: After loading, spin the lower and upper spray arms by hand to confirm clearance.
Keep tall items out of the arm path: Place long utensils flat on the top rack if your model allows, and avoid standing spatulas or whisks upright.
Seat racks fully: Push both racks firmly to their stops; correct any wheel that’s off-track.
Use rack height correctly: Raise the upper rack when washing tall glasses or bowls that could drop into the upper arm’s sweep.
Don’t overload corners: Front corners and edges are common strike zones; keep rims and handles inside the rack footprint.

Conclusion

A dishwasher knocking noise during the wash phase is most commonly the spray arm striking a protruding dish or a mispositioned rack. Confirm it happens during spraying, then do a quick mid-cycle check and a hand-spin clearance test. If the knock disappears with an empty test run, fix the loading pattern; if it persists, suspect a loose or warped spray arm that needs repair.

Frequently Asked Questions

Why does the knocking come and go during the same cycle?

Spray arms speed up and slow down as water pressure changes and as the load shifts. A utensil handle can move slightly, or a lightweight item can reposition, creating intermittent contact. If you can’t reproduce the knock by hand-spinning with the door open, suspect a movable item (lids, light cups) rather than a fixed obstruction.

Can a spray arm hit the rack even if the dishwasher isn’t overloaded?

Yes. A rack that isn’t fully seated, a wheel off-track, or an upper rack set too low can reduce clearance even with a normal load. Another common cause is a spray arm that has loosened and now wobbles, expanding its swing into the rack area.

How do I tell if it’s the upper or lower spray arm making the noise?

The lower arm strike usually sounds deeper and comes from the bottom center, and it often involves the silverware basket or pan handles. Upper arm strikes are sharper and seem higher in the door area. The most reliable method is the pause-and-check: open the door when the knock is happening and look for the best “witness marks” (fresh wet scuffs) near the arm that was rotating.

Is it safe to keep running the dishwasher if the spray arm is knocking?

Occasional contact from a misloaded item is not immediately unsafe, but it can chip dishes and damage the spray arm over time. If the knock is heavy or happens every rotation, stop and correct the interference. If it knocks with no dishes inside, don’t keep running it; continued contact can wear the arm mount and rack coating.

What’s the fastest way to confirm a spray arm is warped?

With the door open and the machine off, rotate the arm slowly by hand and watch the gap between the arm tip and the rack/tub. A warped arm will show a changing gap and may dip low at one point in the rotation. If the arm contacts at the same spot each turn with an empty rack, warping is likely.

After all the noise, it’s oddly peaceful to see the point land where it should. The change feels less dramatic than advertised—more like the day finally behaving itself.

There’s a quiet confidence in that. You don’t have to keep circling the same thought; it just sits there, settled, and gets on with the rest of your day.