Loud humming noise mod#khms155lss0 - KitchenAid KHMS155L Microwave Oven

If your microwave is making an unusual or loud humming noise, you may have a problem with the magnetron. This component is part of the high voltage circuit and provides the microwaves that generate the heat. If the magnetron is defective, it may cause a loud humming or buzzing noise

If your microwave is making an unusual or loud humming noise, you may have a problem with the magnetron. This component is part of the high voltage circuit and provides the microwaves that generate the heat. If the magnetron is defective, it may cause a loud humming or buzzing noise.
https://www.fix.com/repair/appliance/microwave/unusual-noise/

A loud humming noise in microwave mode will usually be a shorted high voltage diode or bad magnetron.

This sound you describe is not normal and should be checked quickly, even if it means returning the item.

May as well use the microwave as is until it terminates naturally. All microwave ovens buzz, that nothing unusual, but although you can replace a Magnetron, the cost is unusually high. By the time you take the oven down, open the case and fuss around, perhaps not making things any better; the fact is; a new replacement with a warranty is just over $100. Why bother?

humming sound is your magneto going out Dont have any idea of the cost for replacement however it may be cheaper to purchase another microwave than to repair this one. Good Luck

Dave

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.

We had this problem with a new GE microwave in our last home. GE service replaced fan assy,.AND added rubber foam around the motor, Loud hum was reduced,but never did completely go away. We learned to live with it/