Microwave makes loud buzzing or humming noise while running, louder than normal, with reduced or no heating.

Safety Warning: Loud buzzing from the high-voltage section can indicate imminent failure. Unplug the microwave and avoid further use until inspected.

User-level checks:

• Inspect the waveguide cover (mica or plastic panel on the side wall) for burn marks or damage. If burned or charred, discontinue use.
• Listen to determine if the noise is from the top/rear (transformer area) or side (magnetron area), but do not remove covers.

Technician-level steps:

• Remove the cabinet and discharge the capacitor.
• Inspect the magnetron and transformer for signs of overheating or arcing.
• Ensure the transformer is securely mounted; tighten mounting bolts if loose.
• Test the high-voltage components and replace the magnetron or transformer as indicated.

Because of the high voltage involved, this repair is best left to professionals.