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Chris: I have a Kenmore Microwave Model No. 721.66222500 and every once in a while it has a ‘hum’ when it gets done with a cycle. Often when you open and close the door, the hum goes away. Sometimes it does not go away but if you leave the door open for 10 min, then close… it goes away. Do other people have the same issue, and if so what is the solution? —

Yes, many other microwave owners experience this exact same issue. On your specific Kenmore 721 series microwave (manufactured by LG), a persistent hum that starts after a cycle ends and disappears when you interact with the door points directly to a failing mechanical or electromechanical component. [1] (https://photos.google.com/link/photo?key=AF1QipM_Y2FY_49bW25-mCdNHroEP0C2uDuioFtltqA&obfsgid=111631210333001349301&utm_source=GM) [1] (https://www.reddit.com/r/Appliances/comments/lercjs/microwave_making_buzzing_sound_when_plugged_in/), [2] (https://forum.appliancepartspros.com/t/humming-when-door-is-closed/307889), [3] (https://www.justanswer.com/appliance/s84hj-humming-when-off-kenmore-microwave-790-80323310-march.html), [4] (https://www.justanswer.com/appliance/5tobb-microwave-buzzes-using-buzzing-stops.html)

Why It Happens

When a microwave cycle finishes, the main high-voltage circuitry (which handles heating) is supposed to turn off, while the internal cooling fan may occasionally run for a moment to clear out steam and cool the components. In your case, the intermittent humming is caused by one of two overlapping issues: [1] (https://www.samsung.com/us/support/troubleshoot/TSG10007187/), [2] (https://www.homeserve.com/blog/home-improvement/microwave-making-noise/)

  1. A Failing Door Interlock Microswitch (Most Common): Microwaves rely on a primary, secondary, and monitor switch cluster behind the door frame. When you close the door, the latches physically click into these plastic switches. If a microswitch has worn out internal springs or slight contact arcing, closing the door firmly can trick the control board. This causes it to prematurely engage or refuse to shut off the internal cooling fan motor or the turntable/stirrer motor. Opening and shutting the door snaps the microswitch back into its correct open state, breaking the electrical path and stopping the hum. [1] (https://www.repairclinic.com/RepairHelp/How-To-Fix-A-Microwave/68-6–/Kenmore-Microwave-Shuts-Off-After-A-Few-Seconds), [2] (https://www.youtube.com/shorts/euDSeMF2jXQ), [3] (https://www.youtube.com/watch?v=G1-ts-VlBYg&t=157), [4] (https://www.justanswer.com/appliance/s84hj-humming-when-off-kenmore-microwave-790-80323310-march.html), [5] (https://www.homeserve.com/blog/home-improvement/microwave-making-noise/), [6] (https://www.justanswer.com/appliance/5tobb-microwave-buzzes-using-buzzing-stops.html)
  2. A Sticking Control Board Relay: The logic board uses small electromechanical switches (relays) to control high current paths. Over time, these relays can suffer from “contact pitting” or become sticky, especially when exposed to heat and moisture from cooking. Leaving the door open for 10 minutes cools down the inner cavity and the circuit board, allowing the relay spring to finally release its magnetic hold and shut off the stuck component. [1] (https://www.youtube.com/watch?v=VQiyKWQg0Zs), [2] (https://www.reddit.com/r/Appliances/comments/lercjs/microwave_making_buzzing_sound_when_plugged_in/)

The Solutions

  1. The Safe, Non-Invasive Fixes (Try First)

• Deep Clean the Door Latches: Food grime or grease buildup inside the door hooks or within the holes on the microwave body where the hooks insert can prevent the door from triggering the switches completely. Clean these areas thoroughly with a degreaser. [1] (https://www.justanswer.com/appliance/5tobb-microwave-buzzes-using-buzzing-stops.html) • Door Realignment: If the screws holding the door or latch assembly are slightly loose, the hook won’t push the internal microswitch cleanly. Check the frame and tighten any loose external screws around the door pivot or latch receiving area. [1] (https://www.ifixit.com/Answers/View/196903/Making+loud+Hum+when+not+on,+but+plugged+in.)

  1. Hardware Replacement (Requires a Professional)

If the behavior becomes more frequent, the definitive solution requires replacing the door interlock switches or the main electronic control board. [1] (https://www.repairclinic.com/RepairHelp/How-To-Fix-A-Microwave/68-6–/Kenmore-Microwave-Shuts-Off-After-A-Few-Seconds), [2] (https://www.youtube.com/watch?v=VQiyKWQg0Zs) • Replacing the Microswitches: A technician will remove the control panel to access the switch holder assembly, test the 3 switches with a multimeter for intermittent continuity, and replace the faulty switch (often a very inexpensive part). [1] (https://www.justanswer.com/appliance/s84hj-humming-when-off-kenmore-microwave-790-80323310-march.html), [2] (https://www.repairclinic.com/RepairHelp/How-To-Fix-A-Microwave/68-6–/Kenmore-Microwave-Shuts-Off-After-A-Few-Seconds) • Replacing the Control Board: If the relays themselves are permanently fusing or stuck closed, the main control assembly must be replaced. [1] (https://www.repairclinic.com/RepairHelp/How-To-Fix-A-Microwave/68-6–/Kenmore-Microwave-Shuts-Off-After-A-Few-Seconds), [2] (https://www.youtube.com/watch?v=VQiyKWQg0Zs)

⚠️ Critical Safety Warning

Do not attempt to open the outer cabinet of the microwave yourself to fix internal electrical components. Microwaves contain a massive high-voltage capacitor that stores a lethal electrical charge (up to several thousand volts), even if the machine has been completely unplugged from the wall for days. Any internal diagnostic work or switch swapping must be handled exclusively by a certified appliance technician. [1] (https://www.beaconsaves.com/blog/microwave-loud-hum), [2] (https://www.repairclinic.com/RepairHelp/How-To-Fix-A-Microwave/78-6–/Kenmore-Microwave-Is-Loud-Or-Noisy), [3] (https://www.youtube.com/watch?v=Xmv9IvBChiw&t=155), [4] (https://www.ifixit.com/Answers/View/378838/Loud+humming+noise+when+cooking+stops+while+door+is+close) How to verify safety: Put a mug of cold water inside, close the door, and let it hum for 30 seconds without starting a cycle. Take the water out. If the water stays cold, only the cooling fan or turntable is stuck running—this is annoying but safe to use until repaired. If the water gets warm, the dangerous heating element (the magnetron) is stuck on; unplug the microwave immediately and do not use it again until it is professionally serviced. [1] (https://forum.appliancepartspros.com/t/humming-when-door-is-closed/307889), [2] (https://www.beaconsaves.com/blog/microwave-loud-hum), [3] (https://www.reddit.com/r/Appliances/comments/lercjs/microwave_making_buzzing_sound_when_plugged_in/), [4] (https://www.justanswer.com/appliance/s84hj-humming-when-off-kenmore-microwave-790-80323310-march.html), [5] (https://www.youtube.com/watch?v=Xmv9IvBChiw&t=155)

Chris: Can you find me the switch partno to order for the green tipped switch? —

The exact manufacturer part number for the door micro-switch on your Kenmore Microwave Model No. 721.66222500 is 6600W1K001C. [1] (https://www.partselect.com/Models/72166222500/) This is the primary interlock switch (often distinguished by a green casing tip or terminal lever in LG/Kenmore configurations). It operates as a normally open (NO) switch that tells the microwave the door is securely shut. [1] (https://www.repairclinic.com/ProductDetail/699203) The part is widely available across major online appliance parts retailers: Kenmore Microwave Door Micro-Switch: Genuine OEM replacement door switch directly sourced from the manufacturer. • Part Number: 6600W1K001C • Replaced By: 3B73361E (Manufacturer official substitute part number) [1] (https://www.partselect.com/Models/72166222500/), [2] (https://www.google.com/search?q=product&prds=pvt:hg,pvo:29,productid:4233102702423066809,catalogid:10279613615620647121,gpcid:5984141658322464537,mid:576462374453210767&ibp=oshop) Would you like assistance finding a certified appliance technician in your area to safely swap the switch, or do you need help tracking down the other two secondary switches in the door latch assembly?