DIAGNOSIS That's the compressor starting, which is normal but can be startlingly loud for the first 24-48 hours of use. It needs to settle in.
FIX Let it run through its initial cycles. Don't interrupt it. The noise should noticeably diminish after the first day or two of continuous operation as the refrigerant and internal components circulate and settle.
IF THAT DIDN'T WORK If the loud grinding persists beyond 48 hours or sounds like metal-on-metal, the unit might be unlevel. Check that all four feet are firmly on a flat, solid countertop. An unlevel unit makes the compressor vibrate excessively.
HEADS UP The first few batches of ice will likely be cloudy or misshapen. Discard them. Clear, solid nuggets only form once the machine is fully cycled and cold.