Over-the-oven microwaves are convenient because they give you more counter space and help ventilate the kitchen. Microwaves are generally noisy when you use them under normal operation but they can sometimes emit hums and thumps that are alarming. Hearing an odd humming sound may be unsettling, but the sound usually is not a sign of trouble. Identifying the humming sound source and determining that the appliance works correctly can help put your mind at ease.
When you use a microwave oven on a setting lower than high, the humming sound may seem more noticeable. According to GE Appliances, the humming sound occurs due to the magnetron tube turning on and off to achieve the lower power setting. The magnetron is part of the high voltage system inside a microwave. The vacuum tube produces the energy that the microwave needs to run. The humming noise produced from the magnetron is normal.
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Hearing a humming sound while you stand in a quiet kitchen may cause you concern. If you hear a humming noise when the microwave receives no power, the transformer may be emitting the sound. The transformer provides power to the clock, and the humming noise will sound subtle. You can hear this sound only if you listen to the appliance closely. If the humming sound is loud, the exterior fan may be functioning.
A microwave has internal and external fans. The internal fan works whenever you use the microwave. The fan circulates the air inside the microwave to help cool the components so the microwave does not overheat. The exterior fans on over-the-oven microwaves often connect to a vent to remove smoke, heat and cooking smells that come from the kitchen cooktop range. If you notice a humming noise while the microwave does not function, turn the fan off by pressing the specific button on the microwave. A fan button usually is indicated with a picture of a fan or it may say "High/Low/Off."
Microwave ovens may make a humming noise for a variety of other reasons. The power diode may be bad, the high voltage capacitor may not operate correctly, or shorted wires may prevent the internal components from working properly. If you cannot pinpoint the humming sound, you may have to consult the owner's manual for specific instructions for the make and model of the microwave. Call the manufacturer directly or call an appliance technician to repair the microwave if you do not have any electrical experience to prevent harm to yourself and the appliance.