GE Spacemaker XL1400 (JVM1451BA) Microwave manufactured Dec. 1998
In the span of a month, there were two times that while cooking (popcorn and just re-heating), there was an extremely loud crack or pop. The microwave continued to function normally... nothing seemed wrong. It definitely was not the food or containers inside that made the noise.
Any ideas what is happening?
Electric Arcing inside the microwave where its power is converted to a very high voltage is the most likely culprit. Based on its age, I would recommend replacing it. Microwaves are dirt cheap these days.
1: The first thing to check is the turntable area is the paint missing from underneath the glass turn table at the rotor - anything burned there.
2: Do you have a porcelain coated metal turntable is any of it scratched off or chipped?
3: You could have lots of food oil / grease in the wave guide and that is why it is arcing .
Your microwave most likely has a bad Magnetron - it is burnt at the tip and the microwave is arcing in the wave guide. About $20 and a half hour of your time to replace it DIY.
Unplug Microwave let it sit for 24 hours before touching it, you can then open it and discharge the capacitor , then disconnect the magnetron and replace it. Clean the wave guide while the cover is off (if the guide is easily accessible), some units on the inside of the Microwave have a plastic like covering that seals the wave guide from this grease and oil - in that case the wave guide might be fine - when you pull the magnetron off check it from the magnetron side as well.
here is a nice guide full of information for general reading. https://www.repairfaq.org/REPAIR/F_micfaq5.html
A Bad Magnetron might look like this one:
