#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.
To give something non-flammable that can absorb the energy, instead of starting a fire. The reasoning being that if you mysteriously see the water boiling, you can unplug the microwave before it starts a fire.
I thought it might be something like that. When microwave ovens were a new idea people had some odd misunderstandings regarding safety.