I'd like to come back to this question - why do microwave ovens make a loud noise?You've had the answer, you just don't want it. The fan makes a loud sound of rushing air, then there's a hum from the transformer and the solenoid around the magnetron.
So far, I haven't seen any convincing answers.
If you switch the oven onto a reduced power you'll hear the timer clicking in and out as it reduces the duty cycle, and you'll hear the hum whilst the magnetron is running, but not when the magnetron is off. Note how the "roaring noise" remains when the magnetron is off: that's because it's coming from the fan and not the magnetron.
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