#Solved
Our microwave has just begun to show a failure mode. With no settings on the timer/modes when we close the door we hear a 60-cycle hum.
We put a coffee cup with water in and closed the door but did not push any button and noted the hum. The water got warm, for the exposure of 20 to 30 seconds, the water got the expected level of warmth we think.
Is there any danger if we leave the door open? (No hum with the door open)
Could the Magnatron be running at a higher output than it is supposed to when the door is closed but the microwave is not under the control of its built-in circuitry?
Of course, there is no danger of the microwave doing anything other than heating stuff (No mad science going on)? We know that microwave ovens work by making the water molecules vibrate faster.
I would definitely unplug it. If you, someone else, or perhaps even a pet accidentally closes the door and it turns on with nobody realizing it bad things can happen. It will basically microwave itself and if you leave it running long enough it creates a big fire risk. It is a small mistake to make with a high risk.
It has been unplugged. I did try unpluging - wait - repluging (reconfiguring the primary power coupling) but that did not resolve the issue.