The oven sparks and gives a loud hum during use with plastic food containers. This scares my wife and mom terribly. I have an Emerson 900W microwave oven (model:MW8999SB).

That loud hum is an indication that the Magnetron is shorted.

Hi,
Here is a tip that will help you to figure out what is wrong with yourMicrowave Oven....

Basic Microwave Oven TroubleshootingTips

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The part that generates microwaves that heat the food is the magnetron. Typically when the magnetron fails, it shorts out. When that happens the oven will produce a loud buzzing or humming sound. It typically not worth the cost to replace the magnetron, because it is almost as much as buying a new oven.

From your description it sounds like the magnetron has failed in this oven. That's the part the makes the microwaves to heat the food. It typically fails by shorting out and making a loud hum noise. It not usually worth fixing since the price of a replacement magnetron is almost as much as a new oven.

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That's what they do - plenty of focused energy at work here. However, try removing the rotating food tray and, with just a glass of water in the oven (never run it empty!) see if the hum goes away. If so, it's the tray driver/carrier.

Brent
The magnatron in your oven has gone out and will need replaced!( This is the device that heats up your food) If everything is working and your food is not getting hot 99.9 percent of the time its the Magnatron or its fuse! The loud humming is also a big clue the MAGNATRON has gone bad!
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You either have a bad magnetron tube (5 or 10 year parts warranty item through GE Factory Service 1-800-432-2737 or www.geappliances.com) or a shorted high voltage rectifier. I think the light being out is a secondary problem.

When talking about "Power Level", there is a misconception that the cooking power is being adjusted. This is not the case. The user is actually adjusting the duty cycle of the Magnetron. For instance at 30%, the Magnetron will be turned on for 20 sec. and off for 40 sec. A loud 60 cycle hum is usually caused by a shorted Magnetron. Thus the unit is not economically repairable.

my guess would be the wave cover. sometimes they smoke and makes sparking sounds. does it still heat food?...if it does, then its not the magnetron.