DIAGNOSIS The cooling fan motor is failing or the fan blade is hitting something. The fan is behind the vent grille on the right side.
FIX Unplug the unit. Remove the vent grille (usually clips). Check for obstructions like a loose piece of packaging or debris. Spin the fan blade by hand. It should move freely and silently. If it grinds or is stiff, the fan motor (part number WB27X10937) needs replacement.
IF THAT DIDN'T WORK The noise could be from the high-voltage diode or magnetron, but this is less common than fan noise. A magnetron hum usually accompanies weak heating.
HEADS UP Do not operate the microwave with a seized fan. It will overheat and can damage the magnetron or start a fire.