Why do many microwave ovens hum in an interval of a minor 7th?

Something that I’ve noticed across most of the microwave ovens that I’ve used is that when they hum while cooking food, I can pick out 2 distinct tones. One of them is pretty clearly 60 120 hz, the 2nd harmonic of the AC power frequency. The other is consistently a minor 7th above that (which would be somewhere around...