Diagnosis
A constant loud hum points to the high-voltage transformer or the cooling fan. On this model, a failing transformer is a common source of excessive noise.
How to Fix It
Unplug the microwave. Check for external obstructions. Ensure the unit has at least 3 inches of clearance on the top and sides for the cooling fan to vent. Vacuum the vent grilles on the top and sides to remove dust buildup. Plug it back in and test. Reduced airflow can cause the fan motor to strain and the transformer to overheat and buzz.
If That Didn't Work
If clearance is good and vents are clear, the noise is likely internal. The transformer laminations can loosen, creating a loud 60Hz hum. The diode or capacitor in the high-voltage circuit could also be failing. These components are not user-serviceable.
Heads Up
A loud buzz that is new and consistent indicates a component operating under stress. Continuing to use it in this state can lead to complete failure. Consider professional diagnosis.