Constant humming sound

It could be the possibility of the bulb about to go out or dust in the fan

If you have that funny burning smell in the microwave oven This is sure sign the magnetron has failed, and must be replaced.

You should first be sure your filters are clean and that your ductwork is clear.

See this link for a bulletin that may address your problem.

Otherwise, see this link in case you have a sticky door switch or relay.

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hi it could be a bad neutral connection on your plug or inside on your high voltage transformer check the plug for burning and try plugging it in a different socket

Sounds like the magnetron bit the dust.

Possibly a shorted relay on the main timer board?? hum coming from transformer being powered with no fan running. With No fan you can hear it hum. shorted power relay on main board.

If your magnetron was shorted, the unit would not heat at all.

The sound of a shorted magnetron is usually a loud hum-- but no or very little smell and no smoke.

Components in the High Voltage section -capacitor, diode- give problems if they short out.

You either have a bad magnetron tube (5 or 10 year parts warranty item through GE Factory Service 1-800-432-2737 or www.geappliances.com) or a shorted high voltage rectifier. I think the light being out is a secondary problem.

Possibly, the "fault" was the buzzing in the first place! Buzzing is usually a cabinet vibration caused by magnetic fields from the "gubbins". If the buzzing should occur again, try pushing/ pulling/ poking the cabinet while the oven is running, you may find a position that causes the buzz to stop.
Another possibility is you previously had something else placed in close proximity to the oven.
If a rattling cabinet is the problem, either try to reposition something, or just ignore it!

my guess would be the wave cover. sometimes they smoke and makes sparking sounds. does it still heat food?...if it does, then its not the magnetron.