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 106-108 212-214 hz depending on the exact ratio). What causes this 2nd frequency to be produced?
Edit: after checking against a tone generator, the low frequency is actually 120 hz, double the grid frequency. The question is still the same, just an octave higher.
Microwaves are fantastic cooking tools, and I'm pretty confident you're not using yours to its potential. Defrosting, reheating, steaming, boiling. Does it all in half the time with half the mess. All those settings on the keypad do something good. Most people just wack a few numbers in and let the microwave literally cremate the food on full power. Those reheat and defrost settings apply microwave then switch to low or no power, leaving the applied heat to radiate internally before repeating. Different densities and starting temperatures are accounted for.
Obviously you wouldn't cook a stir fry or a steak in a microwave. Potatoes before roasting though? Dumplings? Frozens? Yes please Mike.