Why does my GE microwave JVM1339WW-002 run with the door open?

Why does my GE microwave JVM1339WW-002 run with the door open? I'm confident that no microwaves are being generated when this happens because an extra sound is added to the hum of the microwave when the door is closed and running versus when it is open and running. It doesn't need any time setting to do this, just open the door and the fan starts for run. What could be the cause? Can it be fixed at home? The microwave was purchased in 1997.