Diagnosis
Spilled food or grease is burning on the waveguide cover (the mica or cardboard-like panel on the right side wall inside the cavity).
How to Fix It
Unplug the microwave. Remove the glass tray and hub. Inspect the silver-colored rectangular waveguide cover on the right interior wall. If it has splatters, carefully wipe it with a damp cloth. Do NOT scrub or use chemicals. Let it dry completely.
If That Didn't Work
The smell could be from a failing magnetron or wiring insulation, but that's far less common. If the cover is clean and the smell persists during use, the unit has a serious internal fault and should be serviced or replaced.
Heads Up
Never operate the microwave if the waveguide cover is missing, loose, or damaged. It protects the magnetron from food debris and moisture. A damaged cover must be replaced.