Check the condition of your microwave waveguide cover. If it's burned or cracked, it can deflect the microwaves to the metal case, resulting in sparks that could well make a popping noise. Again, I can't give you specific information on the appearance of yours without knowing the model number, but here's a fairly representative picture of what the waveguide cover looks like. it's made of mica, thin and easily breakable.
You'll usually find it on the side of the microwave that has the control panel.
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I also have this problem, just started doing it but its an LG thinQ over range. Im wondering if condensation isnt getting into where the tiny lightbulb is at, I'll note that we dont use the light as it burned out a year ago and I never replaced it
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