Why do many microwave ovens hum in an interval of a minor 7th? - Lemmy.wtf

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 hz from the AC power frequency. The other is consistently a minor 7th above that (which would be somewhere around 106-108 hz depending on the exact ratio). What causes this 2nd frequency to be produced?