If your microwave is sparking when there's no metal in it, the cause of the sparking could be something in the food that could be metallic, chipping paint inside the microwave, a bad magnetron or even damaged waveguide cover.
Also sometimes a cup or bowl might have a metallic decorative ring on the outside which can cause sparking.
The most common reason your microwave is sparking inside and sometimes also may have a burning smell is due to a damaged waveguide cover which is the little paper like rectangle piece located on the inside of the microwave.
When the waveguide cover goes bad or gets damaged it can lead to sparking and a burning smell.
You can replace them pretty cheap.
Other reasons for a microwave sparking and a burning smell is a bad magnetron, diode or bad transformer or other electronics going bad.
When your microwave is suddenly sparking it could be something in the food or something on the dish that could be metallic or even a result of a damaged waveguide cover or even bugs could be inside the microwave and getting into contact with the wires or even the magnetron.
However if the microwave continues to spark and the microwave is actually bad then continuing to use the sparks is not considered safe.
A common culprit of a sparking microwave is a damaged waveguide cover that is located inside the microwave area where you cook the food and it coves the hole where the magnetron sends the actual microwaves.
When that becomes torn or damaged it can result in sparking of the microwave.
You can use your microwave after it sparks as long as the sparking stops and is not a result of a problem with the microwave itself.
If the microwave sparked due to some metal that was on a dish or something in the food caused the microwave to spark or a bug or something else caused the microwave to spark it's generally safe to continue using the microwave.
In some cases the sparking of the microwave can be caused by a damaged waveguide cover inside of the microwave which is fitted over the over the magnetron on the inside of the microwave on the side.
It's a little cover that is similar to a piece of cardboard and sometimes that can result in sparking if it's damaged.
My microwave kept sparking every now and I found out it was the waveguide cover and after removing it, the microwave stopped sparking.
You can replace them easily and cheaply.
If the microwave continues to spark then it could be due to a bad magnetron or other damaged electronics and in that case the microwave should not be used.
If you use a damaged microwave it could result in potential fires, electrical problems and even microwave energy leaks.
If the microwave is damaged, especially if the door, seals or any internal components are damaged the microwave should not be used.
The microwaves that are emitted by the microwave magnetron could escape if the microwave has any damage to it's door, hinges, latches or seals.
Microwave radiation is often safe, but prolonged exposure to the leaking microwaves can result in skin burns or eye damage.
You can place a cell phone inside the microwave with the microwave off with the door closed and if the phone loses it's connection to the network it indicates a potential leak.
When your microwave makes a popping sound it could be due to a bug coming into contact with the magnetron or getting shorted out on some wires inside the microwave.
Or the popping sound from the microwave could also be caused by the food being cooked or the microwave itself.
Food that has a high fat or high water content or foods with skins such as potatoes when cooked in the microwave can pop as the steam escapes from the food.
Even food spills or residue that is left inside the microwave could cause a popping sound from the microwave.
In other cases the popping sound from the microwave, especially if the popping sound continues could be a problem with the fan motor, magnetron or even a bad waveguide cover.
Foods such as sauces, steaks and some vegetables can also pop as they heat up in the microwave as a result of the moisture in these feeds turning to steam.
And foods such as tomatoes, potatoes or even hot dogs, can trap steam in and pop when the pressure builds up as they cook in the microwave and the steam and pressure escapes.
And food splatters or residue inside your microwave can overheat and sizzle or pop.
A failing or faulty magnetron in the microwave can also produce popping sounds or buzzing sounds as the magnetron is what generates the microwaves that heat and cook the food.
A defective diode in a microwave which also works with the magnetron can also cause popping sounds.
And even the turntable motor that rotates the food and the cooling fan that helps regulate the temperature could make popping and scraping sounds.
If your microwave has any arcing it can be a result of a damaged or dirty waveguide cover or also a faulty magnetron which can create popping or sparking sounds.
Or if you happen to use a metal utensil or foil in the microwave it can create arcing and popping.
In some cases I've had my microwave pop and found out it was a bug inside the microwave that made contact with the wires or the magnetron.
If it continues then the microwave may be failing.