DIAGNOSIS That's almost always the metal crisper plate (the perforated tray that sits in the basket) vibrating. It doesn't sit perfectly flat or has gotten slightly bent.
FIX Take the basket out and remove the crisper plate. Lay it on a flat countertop and press down gently on any high spots. Even a slight warp causes noise. When you reinsert it, make sure it's seated fully and evenly in the grooves of the basket.
IF THAT DIDN'T WORK The noise could be the internal fan hitting a loose piece of food debris or a broken blade. Unplug the unit, turn it upside down, and gently shake it over the trash to dislodge any crumbs. If you still hear metal-on-metal, the fan assembly is likely damaged.
HEADS UP Never operate it without the basket and crisper plate properly installed. The fan is designed to pull air through that specific setup, and running it empty can cause overheating and make any imbalance worse.