Diagnosis
A foreign object is caught in the wash arm or pump assembly. The noise is metal-on-metal contact from the wash arm striking debris.
How to Fix It
Remove the lower rack. Manually rotate the lower wash arm. Check for obstruction. Remove the wash arm by turning the retaining nut counterclockwise. Inspect the sump area beneath it for utensils, glass fragments, or bone. Clear all debris.
If That Didn't Work
The noise may originate from the drain pump. Remove the kick plate and access the drain pump housing on the lower right side. Check for a small object lodged in the impeller.
Heads Up
The most common culprits are olive pits, cherry stones, and broken glass. Run a rinse cycle with the wash arm removed to flush the sump before reassembly.