DIAGNOSIS That's the compressor kicking on for the first time. It's a normal, albeit startling, sound for these portable units when they've been shipped on their side or upside down.
FIX Unplug it immediately. Leave it upright and untouched for at least 8 hours, ideally 24. This lets the refrigerant oil settle back where it belongs in the compressor. After the wait, plug it in, press the power button, and select your ice size. The startup should be much quieter.
IF THAT DIDN'T WORK If the grinding persists after the settling period, it could be a fan blade hitting something. Check the rear vent for any obvious plastic packaging debris that might have gotten sucked in.
HEADS UP Always let these things sit upright after moving them. Tilting or laying them flat stirs up the compressor oil and the unit will sound like it's dying for a few cycles until it redistributes.