Diagnosis
The fan motor is hitting an obstruction. It's usually a loose piece of food or a small bone caught in the fan blades inside the top heating assembly.
How to Fix It
Unplug the unit and let it cool. Remove the basket. Look up into the heating element chamber. Use a flashlight. You'll see the circular fan. Use long tweezers or needle-nose pliers to carefully remove any debris from between the fan blades.
If That Didn't Work
The fan motor shaft might be damaged or the bearing is failing. Listen for a grinding metallic sound. This requires a replacement fan motor or top assembly.
Heads Up
Do not operate it with this noise. The fan is critical for airflow and preventing overheating. A stuck fan can blow the thermal fuse.