No heat in microwave. Makes loud humming sound when turn on. Model GE Spacemaker JVM166OWBC 04

The humming can be caused by a couple of bad parts. Usually it's the magnetron tube, the microwave source. They often fail by shorting out, and the loud hum from the power supply is what you hear. Another part called the HV diode may also have shorted, but the magnetron is more common.

A bad magnetron can be replaced, but it's a costly part. And microwave service should be left to a professional, because there are dangerous voltages under the cover and a chance of microwave leakage if repairs aren't done correctly. Between the part and labor costs, it might be more economical to replace the oven. Let a qualified servicer give you a repair estimate and then decide.