What Are the Causes of Knocking in a Dishwasher?

While dishwashers cut down on cleaning time and free you from the kitchen sink, they are certainly not perfect -- these sometimes noisy appliances hum when the water inlet valve opens, purr as the motor kicks in and snap each time the solenoid opens during a cycle. Although these sounds are harmless, knocking noises typically call for a bit more attention. Numerous issues -- some benign and some that indicate a need for repairs -- can cause these sounds; knowing what to look for puts you one step closer to eliminating the noise.

If your dishes aren't properly arranged, they may hit the appliance's sprayer arm during operation, producing a regular, rhythmic thump. To ease this problem, load your dishes so that nothing touches the interior walls of the unit or interferes with the rotation of the sprayer arm. Ensure that the dishes do not touch each other or protrude from the bottom of the racks. In addition to reducing noise, keeping the sprayer arm free of obstructions also improves its cleaning effectiveness.

An improperly leveled dishwasher can cause excessive dish vibration and make them bang against each other. Remove the dishwasher's bottom-front access panel to access its leveling legs. Open the washer's door and place a level on the rack railing, which runs vertically across the interior compartment, and then place it horizontally across the open door to ensure that the machine is level. Adjust the leveling legs by turning their screws with an adjustable wrench or by enlisting a partner to lift the appliance as you adjust the legs. Turn the dishwasher off and unplug its power cord before leveling.

If your dishwasher produces a metallic clank or clunk -- sometimes closer to a tinny, sharp rattling sound -- the water pipes connected to the appliance may be at fault. As kinetic energy builds up in the pipes, and the water flow stops abruptly, it creates a shock wave that hits against the pipes. Known as water hammer, this event makes a disturbing sound. For water hammer knocks, contact a plumber to install a water-hammer device in your plumbing.

If your dishwasher only knocks when it fills with water, this usually indicates a water-inlet valve in need of replacement. Sometimes, completely benign issues cause knocking noises. For instance, when your dishwasher's detergent cup opens, you may hear a single knock or clanking sound, typically at about halfway through a complete cycle, which is a normal sound.