system
Given the following post, respond as if you're commenting on Reddit:
human
Why do microwave ovens make such a distinctive humming sound?

When I look this up the only answers I come across either talk about the beep sound or just say the fans are powerful.

But I can't find out why they all make the *same* distinctive humming noise, surely it should differ from manufacturer to manufacturer? Surely some brands would want to use quieter fans?
gpt
The hum is 60Hz (or 50Hz if you live in Europe), it’s either oscillations in the components of the rather massive power supply required to feed nearly 1.5kW to the magnetron. Or mechanical oscillations induced in the magnetron due to ripple in its power supply. 

Electronic components can produce mechanical vibrations. Power transformers have to deal with changing magnetic fields that will produce torques in a similar way to a motor. Ceramic capacitors tend to be piezoelectric and mechanically distort with changing voltages. To provide 1.5kW to anything large fields are involved, and it becomes rather hard to provide stable power.

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Edit: As it was pointed out by several people (and confirmed by measurements from others (isn't it nice how science works)). the actual fundamental frequency is twice the line frequency. So that's 120Hz for the US (and related areas) and 100Hz for Europe (and related areas). Those roughly correspond to a B3 and a G2 in the music scale.

There are multiple reasons for this, the main one among them is that the movement of components (such as the microwave's metal case and transformer core and coils) are affected by the magnitude of the magnetic field not its direction, which leads to rectification of mechanical displacement at twice the cycle rate.