Diagnosis
The fan blade is hitting something. On the 6.3QT model, the fan is mounted directly to the motor shaft behind the heating element.
How to Fix It
Unplug and let it cool completely. Remove the basket. Look up into the heating chamber. Use a flashlight. You will see the circular fan. Use long-nose pliers or tweezers to remove any debris (like a piece of food or packaging) caught between the fan blades and the housing.
If That Didn't Work
The fan blade itself may be warped from heat or impact. This requires disassembly to inspect. The fan is not a user-serviceable part. Contact Chefman support for a replacement heating/fan assembly.
Heads Up
Never operate the unit with this noise. A stuck fan causes overheating and can damage the motor or control board.