Diagnosis
The fan impeller in the top cooking zone is hitting an obstruction or its bearing is failing.
How to Fix It
Unplug the unit. Remove the top basket and crisper plate. Look up into the heating chamber. Use a flashlight. You will see the circular fan guard. Use long tweezers or compressed air to remove any loose food debris (like a piece of popcorn kernel or breading) caught between the fan blades and the guard.
If That Didn't Work
The fan motor shaft may be warped or the impeller may have come loose. This requires disassembly to inspect. Not user-serviceable. A persistent grinding noise means the motor needs replacement.
Heads Up
Do not operate with a grinding fan. It can seize, causing the motor to overheat and potentially trip the thermal fuse, leaving the unit dead.