DIAGNOSIS The noise is from the magnetron cooling fan or a vibrating component. A loud, consistent hum points to the high-voltage diode or transformer.
FIX Check for foreign objects (like a twist-tie) caught in the fan intake vents on the right side of the cabinet. Remove any obstructions. Ensure the microwave is on a level surface. Run it empty for 30 seconds to see if the noise persists.
IF THAT DIDN'T WORK The internal mounting bolts for the magnetron or fan may have loosened. This requires disassembly. For safety, do not proceed unless you are qualified. The symptom is a pronounced 60Hz buzz.
HEADS UP A loud buzzing that starts and stops with operation, not just fan noise, indicates an electrical component failure. Continued use in this state can lead to complete failure.