Our KitchenAid microwave works well, heats food, etc., but makes a constant loud and annoying humming noise (close in frequency and tone to the hum of cold fluorescent tubes) even when not in use. When the humming noise first started (around six months ago) I started unplugging the appliance from the wall when it began humming. When plugged back in the hum was inaudible for a while, but would slowly increase in volume over a period of hours. This time period got progressively shorter. Now the unit hums whenever and as soon as it's plugged in. Thanks for any opinions.

Whirlpool Microwave Buzzing Loudly - ApplianceBlog

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Aug 12, 2011 - 10 posts - ‎2 authors
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A loud humming noise in microwave mode will usually be a shorted high voltage diode or bad magnetron.

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

Basic Microwave Oven Troubleshooting Tips

heatman101

A loud humming or buzzing noise would indicate a short on the magnetron circuit, the part that heats the food. These can be expensive to repair since the cost of the magnetron can be as much as a new oven. Get a few estimates before deciding whether to repair or replace.

If it is making a loud noise, check the diode and magnetron for damage.
BEWARE OF LETHAL VOLTAGES!
The fault may be just the diode and a reasonable cost will be involved. If it is the magnetron, depending upon where you purchase one from will command the price.
Hope that helps.

Hi,

Here is a tip that will help you to figure out what is wrong with your Microwave Oven....

Basic Microwave Oven Troubleshooting Tips

heatman101

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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.

If it does not heat, with all other functions working, the problem lies with the magnetron circuit. The magnetron is the part that makes the microwaves to heat the food. Typically, the magnetron fails and shorts out. Symptoms when this occurs is a loud hum from the oven and does not heat the food. The replacement magnetron costs are close to the price of a new microwave oven, so it rarely pays to have it fixed.

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.

If all of your door switches check out, replace the magnatron, capacitor, and diodes.