DIAGNOSIS A foreign object is likely interfering with the fan blade located at the top of the cooking chamber.
FIX Unplug the unit and let it cool completely. Remove the basket and drawer. Inspect the heating element and fan area at the top of the chamber. Use a flashlight. Look for loose packaging material, a stray piece of food, or a small object. Remove any debris with long-handled tweezers.
IF THAT DIDN'T WORK The fan motor shaft or bearing may be defective. The grinding noise would be mechanical and constant. For a new unit, this is a manufacturer defect. Contact Cosori for a replacement.
HEADS UP Never insert utensils or spray cooking oil directly upward into the chamber. Oil can drip onto the fan motor, attracting debris and causing noise or failure.