Diagnosis
The cooling fan motor bearings are failing or the fan blade is hitting something. The fan is behind the rear grille.
How to Fix It
Unplug the unit. Tilt it forward and look up into the base through the intake vents. Use a flashlight. Check for obstructions like a food particle or loose piece. If clear, the fan motor assembly is the cause.
If That Didn't Work
The fan motor shaft bearings are worn. The motor is a sealed unit. You must replace the entire cooling fan assembly. Cosori can provide the part number.
Heads Up
A failing fan will lead to overheating. Do not ignore this noise. The unit may eventually trip on overheat and shut down mid-cycle.