DIAGNOSIS Hardened food splatter on the mica cover. This is the cardboard-like sheet on the inside top of the cavity. It protects the waveguide.
FIX Put a microwave-safe bowl of water with lemon juice or vinegar inside. Run for 5 minutes to steam the interior. Let it sit for 10 minutes. Wipe with a soft cloth. For stubborn spots, use a plastic scraper, not metal.
IF THAT DIDN'T WORK If the mica cover is stained or has arcing/burn marks, it must be replaced. Part number WB27X11036. A damaged cover can cause sparking.
HEADS UP Never use abrasive pads, steel wool, or harsh chemicals like oven cleaner on the interior. They will damage the coating and the mica cover.