DIAGNOSIS That's usually the evaporator plate—the cold metal part the ice forms on—getting hung up during the twist-and-release cycle. A piece of ice or scale is jamming the mechanism.
FIX Unplug the machine immediately. Open the top lid and look at the silver evaporator plate. Manually turn the plastic gear or center shaft (you can usually grip it with your fingers) to rotate the plate through a full cycle. This will often dislodge the stuck ice chunk. Once it moves freely, plug it back in. It should re-home itself and the noise should be gone.
IF THAT DIDN'T WORK The noise could be from the fan hitting a wire or debris. Unplug it, tip the unit back (carefully, so no water spills from the internal tank), and look through the bottom grille. Use a flashlight to see if the fan blade is obstructed by anything.
HEADS UP A constant low hum is normal—that's the compressor. The loud grind is a distinct, alarming metal-on-plastic sound. Don't let it keep running like that; you can strip the plastic gears.