DIAGNOSIS That's almost always the high-voltage diode going bad. It's a small, cylindrical part that helps the magnetron work, and when it starts to fail, it can vibrate or 'sing' loudly under power.
FIX Safety first: Unplug the microwave and discharge the capacitor as described in the F0 fix. Locate the diode—it's attached to the capacitor and the magnetron, looks like a little black barrel with a wire on each end. It'll be mounted in a small holder. Remove it and test it with a multimeter set to the diode setting. A good diode will show continuity in one direction and not the other. If it's shorted (continuity both ways) or open (no continuity either way), it needs replacing. It's a cheap part.
IF THAT DIDN'T WORK If the diode tests fine, the noise could be coming from the magnetron itself, or sometimes the cooling fan motor if a blade is hitting something. With the unit unplugged, spin the fan blade by hand to see if it grinds or catches.
HEADS UP Never, ever operate the microwave with this loud buzzing noise for long. A failing diode can put extra strain on the magnetron and lead to a much more expensive failure. Fix it as soon as you hear it.