All of sudden my 9 year old microwave produced burning sound and smell just a minute or so into the operation, and after turning it off there are a couple of black spots (size of a penny) on the inside of the microwave door. There was nothing unusual in the microwave at that time, just the type of food that we had reheated many times before. What could possibly be the culprit? Repair is unlikely, but still curious to know what could have caused this ...
For some reason I read past "9 year old microwave"... well here you go:
Or this: (Personally, I'd be a little less lax on saying, eh... it's fine)
Sparking and scorch marks in microwave. What's going on here?
It's possible that either the glass or the [frozen food] allowed charge to build up to the point of arcing. It's also possible that there is something wrong with the microwave. Try putting a cup of water in the microwave and running it for about 30 seconds. If there is no arcing then the microwave itself is probably fine. If you continue to have arcing then replace the microwave.
Microwaves don't like ice. -thenakedscientists.com