Diagnosis
That's the sound of the wash arm assembly grinding against something hard trapped underneath it. A piece of glass, a bone, or a utensil has fallen through the lower rack and is interfering with the spray arm's rotation.
How to Fix It
1) Cancel the cycle and let everything cool down if it was running hot. 2) Remove the bottom rack completely. 3) Look at the center of the tub floor—you'll see the wash arm assembly. Manually rotate it. You'll likely feel it catch on something. 4) Inspect the area underneath the wash arm and the filter assembly. Remove any debris you find. Also check that the wash arm itself isn't cracked and catching on its own mount.
If That Didn't Work
The noise could be coming from the wash arm's drive shaft or motor coupler. With the power off, try to move the wash arm up and down. If there's excessive play or it feels crunchy, the internal motor assembly might be damaged.
Heads Up
Never ignore this noise. Running it with something jammed can strip the plastic gears in the drive system, leading to a much more expensive repair.