DIAGNOSIS A constant, loud hum or buzz that's distinct from the normal fan and turntable motor sounds usually points to the high-voltage diode or the transformer. With Panasonic's inverter technology, a failing inverter board can also produce unusual humming.
FIX First, rule out vibration. 1) Make sure the microwave is level on its feet and not touching the wall or cabinets. 2) Check that the glass turntable and ring guide are properly seated and not cracked. If the noise persists, it's internal. This is not a fix for most homeowners. The high-voltage components are dangerous. Your options are to live with the noise if the microwave still heats properly, or to have it serviced. A technician can diagnose whether it's a diode, transformer, or the inverter board.
IF THAT DIDN'T WORK If the humming is new and accompanied by slower heating times or sparking inside, stop using the microwave immediately. This indicates a more serious fault in the high-voltage circuit (like a shorted capacitor) that could lead to component failure or a fire risk.
HEADS UP All microwaves have a normal operational hum from the transformer and cooling fan. What you're listening for is a significant change in volume or tone. If it suddenly gets much louder, it's a sign of stress on the electrical components.