Microwave makes loud buzzing or humming noise when heating, louder than usual.

Safety warning: Loud buzzing from the high-voltage area can indicate arcing or imminent component failure.

User-level checks:

• 1. Confirm source: Determine if the noise occurs only when heating (not in standby). If it stops when you open the door, it is related to the HV system.
• 2. Check load: Avoid running the microwave empty; always test with a cup of water.
• 3. Listen for vibration: If the noise sounds like rattling, the transformer or magnetron mounting screws may be loose, but this still requires cabinet removal.

Recommendation: Stop using the microwave and have a Toshiba technician inspect the transformer, magnetron, and HV wiring for arcing or mechanical looseness.