Diagnosis
That's usually the high-voltage diode or the magnetron itself starting to fail. The diode converts power for the magnetron, and when it goes bad, it can cause a loud, rhythmic buzzing.
How to Fix It
Listen closely. If the buzz is a loud, steady 60Hz hum, it's more likely the diode. If it's a loud arcing or crackling sound, it could be the magnetron arcing internally. For the diode, you'd need to open the unit (UNPLUGGED AND CAPACITOR DISCHARGED), locate the diode (it's a small, cylindrical component near the capacitor), and test it with a multimeter. A good diode will show continuity in one direction only.
If That Didn't Work
The magnetron might be the source. A failing magnetron can buzz loudly and will often lead to weak heating performance. Replacing a magnetron is a standard repair but requires handling the high-voltage components safely.
Heads Up
A loud buzzing accompanied by a burning smell or sparks is an immediate stop-use-and-unplug situation. It indicates serious arcing that can damage other components or be a fire risk.