Diagnosis
The convection fan blade is warped or the motor shaft is bent. This happens from thermal stress or an impact.
How to Fix It
Unplug the unit. Remove the rear cover (several Torx T15 screws). Inspect the plastic fan blade mounted to the motor. Look for cracks or melted spots. Manually spin it to see if it wobbles. A warped blade must be replaced.
If That Didn't Work
If the blade looks perfect, the bearing in the fan motor itself is worn out. The whole motor assembly needs replacement.
Heads Up
Running it with a grinding fan can overload the motor and burn out the main board. Stop using it until fixed.