Diagnosis
The noise is from the high-voltage transformer. It can buzz loudly if the line voltage is low, or if internal components are vibrating against the chassis.
How to Fix It
Check for resonant vibration. Ensure the unit is level on the countertop. Place a non-slip pad or damp towel underneath it. Press down lightly on the top and sides while it operates. If the noise changes, something is loose.
If That Didn't Work
The magnetron or transformer mounting bolts may have loosened. This requires disassembly. Do not proceed unless you are qualified. The unit stores a dangerous electrical charge even when unplugged.
Heads Up
A loud, persistent buzz accompanied by weak heating or sparking indicates a failing high-voltage component. Discontinue use and seek service.