Diagnosis
The motorized damper or the fan impeller is obstructed. Grinding is typically a foreign object or a loose component contacting the fan blades.
How to Fix It
Remove the grease filter(s). Shine a flashlight into the vent opening. Look for a piece of packaging material, foil, or a utensil. Use long needle-nose pliers to remove any debris. Visually inspect the fan blades through the opening for damage.
If That Didn't Work
The damper motor gearbox may have failed. The damper is the flap behind the front grille that opens for external venting. A stripped gear causes a loud grinding. This requires removing the microwave to replace the damper motor assembly.
Heads Up
Operating with a grinding noise can damage the fan motor. Run the fan only on low speed for temporary use until the obstruction is cleared.