There are several spinning/turning components that can cause loud noise when you microwave is tuned on.
Fan Motor
If you microwave start making noise as soon as it turned on prior its operation, then the noise is coming from Fan Motor or Fan Blade. The fan is the first thing that starts spinning as soon as the microwave turned on.
Magnetron
If your microwave is emitting high pitched or growling noises during operation, the magnetron could be going bad. Normally a nearly silent part, the magnetron uses high voltage and induces the frequency required to cook food. Although normally not a safety hazard to operate, this is a sign that the magnetron is failing and will need to be replaced as soon as possible.
Exhaust fan motor makes loud humming noise
Stirrer motor
Another leading cause of a noisy microwave is an expired stirrer motor. With use this part becomes worn and makes a loud grinding noise, which is usually a leading indicator that the part needs to be replaced. The stirrer is a metal blade that is slowly turned by the motor. This action disperses the microwave energy in a random pattern, which heats the food evenly. If the stirrer motor is determined to be nearing expiration, it should be replaced to halt the grinding noise.
Turntable motor
A very common cause for a noisy microwave is a worn out turntable motor. This is a relatively easy repair and will put an end to the noise once replaced.
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