Diagnosis
A small piece of broken glass, a bone, or a hard pit is caught in the wash arm or the chopper/grinder assembly at the bottom of the tub.
How to Fix It
Unplug the dishwasher. Remove the bottom rack. Manually rotate the lower spray arm and check its holes for blockage. Then, look into the small, round plastic grinder cover (impeller) at the very bottom center of the tub. Use needle-nose pliers or tweezers to carefully remove any visible debris. Don't pry off the cover.
If That Didn't Work
A utensil (like a thin knife or skewer) could have fallen through the rack and is hitting the spray arm. Remove all racks and inspect the entire tub floor and the space around the spray arm stem.
Heads Up
If the grinding noise is a constant metallic screech or rumble, the wash pump motor bearings might be failing. Stop using it and call for service to prevent a flood.