DIAGNOSIS The cooling fan motor bearings are failing or the fan blade is hitting something.
FIX Unplug and cool. Tilt the unit and look through the rear air intake grille. Use a flashlight to see if any food debris (like a piece of onion skin) is lodged in the fan blades. Use long tweezers to remove it.
IF THAT DIDN'T WORK The fan motor is likely defective. The fan is attached to the main housing. This is not a user-serviceable part. You need to contact Cosori for a replacement unit or authorized repair.
HEADS UP Do not operate the air fryer with a grinding or scraping noise. A failing fan can overheat the electronics and is a fire risk. Stop using it immediately.