Diagnosis
Arcing in the high-voltage compartment. The most common cause is a buildup of grease and food spatter on the waveguide cover (the rectangular mica plate inside the cooking chamber on the right wall).
How to Fix It
Unplug the microwave. Remove the waveguide cover—it's held by one or two screws. Clean it thoroughly with warm, soapy water and dry it completely. Wipe the interior cavity wall behind it clean of any debris. Reinstall the cover securely.
If That Didn't Work
If cleaning doesn't stop the arcing, the high-voltage diode or capacitor could be failing. This requires a technician to test and replace those components.
Heads Up
Never operate the microwave with the waveguide cover removed or damaged. That directs microwaves into the components and can cause severe damage and fire. If the cover is warped or burned, replace it.