DIAGNOSIS That's the auger motor struggling, usually because the ice in the bin has fused together into a solid block, jamming the mechanism.
FIX Turn the machine off and unplug it. Open the top and look into the ice bin. If you see a solid mass of ice, you'll need to break it up. Use a plastic utensil or the handle of a wooden spoon—never metal—to gently fracture the ice block. Remove all the loose ice. Plug it back in and turn it on. The noise should stop.
IF THAT DIDN'T WORK If the grinding happens even with an empty or nearly empty bin, the problem might be internal. Sometimes a piece of the plastic ice chute inside the machine breaks off and gets caught in the auger. This requires disassembly to check, which isn't a user-serviceable part on this model.
HEADS UP That grinding is bad for the motor. To prevent it, don't let the ice bin sit full for days on end. The ice will sinter together, especially in a warm kitchen. Use or empty the bin every couple of days if you're not making ice constantly.