dishwasher is making a loud grinding or rattling noise during the wash cycle, sounds like it's coming from the bottom.

Diagnosis

The grinding or rattling noise in a Bosch 800 Series SHPM88Z75N is almost always caused by a foreign object bypassing the filtration system and entering the impeller housing of the circulation pump. Because this model uses a high-efficiency pump system, even a tiny fragment of glass, a popcorn kernel, or a fruit pit can lodge between the impeller blades and the pump housing wall. When the motor spins at high RPMs during the wash cycle, this object is struck repeatedly, creating a loud, metallic rattling or a consistent grinding sound that vibrates through the chassis.

Another common cause is a "spray arm strike." If the lower spray arm is slightly warped or if a tall utensil has fallen through the rack, the arm may be hitting the bottom of the tub or the dishwasher basket during its rotation. You can confirm the pump theory if the noise persists even when the spray arms are manually blocked from spinning. If the noise is rhythmic and coincides with the rotation of the arm, it is a mechanical obstruction. If the noise is a high-pitched scream or a jagged grinding, the impeller is likely compromised or obstructed.

How to Fix It

Follow these steps to clear the pump and inspect the mechanical components:

1. Power down the unit and cancel any active cycles. Open the door and remove the lower rack completely to gain full access to the tub floor.
2. Rotate the cylindrical microfilter counter-clockwise and lift it out. Remove the flat mesh filter. Set these aside and rinse them under a faucet to ensure they aren't clogged.
3. Reach into the sump (the hole where the filter was) and feel around the perimeter of the pump intake. Use a flashlight to look for shards of glass, toothpicks, or plastic ties. Use a wet/dry vacuum to suck out any standing water and debris from the sump.
4. Locate the pump cover (the small plastic cap over the impeller). On the SHPM88Z75N, this can often be popped off with a flat-head screwdriver or a finger depending on the specific production run. Remove the cover and inspect the impeller blades. If you see a foreign object, use needle-nose pliers to extract it.
5. Manually rotate the impeller with your finger. It should spin smoothly with a slight "springy" resistance. If it feels gritty or catches, there is still debris inside the housing.
6. Reinstall the pump cover, lock the microfilter back into place, and replace the flat filter.
7. Run a "Quick" cycle with no dishes to verify the noise has ceased.

If That Didn't Work

• Check the Drain Pump Impeller: The noise may be coming from the drain pump rather than the circulation pump. This occurs specifically at the beginning or end of a cycle. You must remove the sump cover and the drain pump assembly (usually held by a single Torx T20 screw) to inspect the small impeller for seeds or glass shards that have bypassed the primary filter.
• Inspect the Lower Spray Arm Bearings: Remove the lower spray arm by pulling it upward. Check the plastic mounting point for wear or cracks. If the bearing is worn, the arm will tilt during operation and rattle against the tub. Replace the spray arm assembly if you see any splitting in the plastic.
• Verify Leveling and Installation: If the unit is not perfectly level, the tub can flex slightly during high-pressure cycles, causing the internal components to vibrate against the cabinet. Use a bubble level on the top frame and adjust the front leveling legs until the unit is dead-center.
• Replace the Circulation Pump: If you have cleared all debris and the grinding persists, the pump bearings have likely failed. This requires removing the side panels and replacing the entire heat pump/circulation motor assembly (Part #00665120 or similar depending on the build date).

Heads Up

• Avoid Pre-Rinsing Glass: While it seems counterintuitive, "pre-washing" dishes can actually loosen small shards of broken glass that then slide under the filters and into the pump. Simply scrape large debris off plates.
• Check for "Rattling" Utensils: Ensure that long knives or spatulas are not protruding through the bottom of the silverware basket, as they can strike the lower spray arm and mimic a pump failure.
• Filter Maintenance: Clean the microfilter once every two weeks. A clogged filter forces the pump to work harder, increasing cavitation and noise levels.
• Avoid Heavy Detergent Overuse: Using too much detergent can cause excessive foaming, which can lead to pump cavitation—a sound that often mimics a grinding or rattling noise.