Microwaves hum normally, but a new, loud hum signals a component under strain.
Failing Magnetron
A magnetron near end of life often hums loudly and heats poorly.
Fix: A technician tests and replaces it.
Drive or Cooling Fan
A worn fan motor or turntable motor can buzz loudly.
Fix: These motors are replaceable parts.
High-Voltage Diode
A failing diode can cause a loud buzz with arcing.
Fix: This requires professional diagnosis and safe capacitor discharge.
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