DIAGNOSIS That's usually the transformer humming excessively. It can happen as the microwave ages, or if it's operating on a slightly low voltage from your outlet. It's often more pronounced at lower power settings.
FIX First, ensure it's on a dedicated outlet or one not sharing a circuit with other high-draw appliances (like a toaster or coffee maker). Try running it at full power (100%) for 30 seconds. Sometimes this can settle the transformer laminations.
IF THAT DIDN'T WORK The noise could also be a worn cooling fan motor. The fan is behind the vent grille on the side. If the buzzing has a grinding quality, the fan motor bearings might be failing, which will eventually lead to overheating.
HEADS UP While annoying, a consistent hum isn't always a critical failure. But if the hum suddenly gets much louder or is accompanied by a burning smell or sparks, stop using it immediately—that points to a serious electrical fault.