Diagnosis
The drain pump impeller has likely ingested debris like glass, bone, or a utensil. The plastic impeller is grinding against the housing or a foreign object.
How to Fix It
Disconnect power. Remove the lower kick plate. You will see the drain pump assembly at the bottom center. Disconnect the hoses and wiring harness. Remove the mounting screws. Inspect the pump impeller by rotating it. Remove any debris. Replace the entire pump (part WD21X25402) if the impeller is broken.
If That Didn't Work
If the noise persists with a clear pump, check the wash pump/motor assembly. A failing wash motor bearing can also cause grinding, but usually during wash, not just drain.
Heads Up
Always check the filter and sump area for debris first. Running the dishwasher with a damaged drain pump can flood your kitchen.