Hello XXXXXXXXX,
Welcome to Just Answer.
Sorry to hear about your problem and thanks for waiting.
You will need to look at and/ or test these three areas if your microwave won't heat but you hear a loud buzzing or humming noise while it is running:
Click here to learn more about each one of these items and to learn how to test them and replace them if necessary.
If the microwave sounds fine and there is no heat, one of the internal door switches, the high voltage circuitry, or the circuit board may be the problem and should be tested and replaced as necessary.
CAUTION: Before attempting any repair, discharging your microwave's capacitor is absolutely essential in preventing injury to your microwave, your tools, and yourself. A capacitor stores large amounts of electricity even when your microwave is unplugged, and it must be discharged before beginning any repair.
If you need further help or clarification on this answer, please do not hesitate to reply to this post.
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Best of Luck, Brian
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Handyman
12 years specifying, researching & troubleshooting appliances. I install and fix all my own.