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To avoid personal injury or death, disconnect your appliance from its power source before you start any troubleshooting or repair work. Use caution when working inside any appliance.
If you hear a low humming sound while the microwave is running, it's probably the magnetron making the sound. Unless the microwave is set to high, the magnetron cycles on and off while cooking. Sometimes the cycling may sound more like a thump depending on your model microwave.
The other reason for a humming sound is because there is a transformer inside the case. This reduces power from the 110V feed to a level sufficient to operate the clock and light in the microwave. This particular humming sound will be noticeable when the microwave is not running.
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