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Frigidaire SxS started having problems cooling on the refrigerator side, but freezer seemed fine. There was an intermittent, quiet hum that sounded like it was coming from the mid/top freezer side. Every couple of hours it would hum/buzz quietly for about 3 seconds, then pause for 5-10 seconds, then repeat. Sounded like when the water pump starts, but came from lower down on the freezer side and was much more brief and repetitive.
Ran through the checklist of things to check...Compressor, compressor fan working. Pulled off the interior freezer panel and evaporator fan working (seemed in constant run tho). Evap coils were iced over a bit, so defrosted them, replaced the panel, and plugged in the unit. Immediately, the intermittent buzz started again and I could tell it was coming from the area of the coils. Placing my hand on the panel and pressing, I could feel the coils vibrating every time the buzz repeated.
Do I need to replace the coils themselves, or could it be something else?
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Rank: Member
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Replacing the evap isn't really an option for anyone but the most hardcore DIYer, you'd need to borrow/rent/buy equipment that is specialized to refrigeration/HVAC.
From your timing description I'd say you are hearing the damper in the top left corner of the FF (fresh food) side opening and closing. This part allows cold air from the FZ (freezer) to blow through to cool the FF. All the cooling happens in FZ side. My guess is the damper or defrost control is bad. This has an adaptive defrost control (ADC) board that can fail and leave it in a state between defrost and normal run. Usually leaving the compressor running and maybe the defrost element on as well until the defrost thermostat opens. The board may try to reset itself and each time it opens and closes the damper. Same as if you unplugged it and plugged it back in after a few seconds. I think it also closes the damper to isolate the FZ during defrost.
Verify that the damper is what you hear opening and closing. You can do this by shining a flash light into the top right corner of the FZ and seeing if it changes the lighting in the FF side as you hear the noise. You'll probably have to tape door switches in to tell the machine the doors are not open.
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Rank: Member
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Thanks for the quick feedback, Drew. Actually had the damper part already on order based on some additional research around cold FZ / warm FF. Replaced the damper as soon as it arrived earlier today, but the intermittent hum persists.
Still, I wanted to check and see if it was in fact the damper motor/door alternating off/on open/close, so I used your tip on the flashlight technique. Taped down the door switches and shined the light, which shined through without interruption. So, seems the damper is in constant "open" position.
Went ahead and pulled the freezer int rear panel back out and was able to confirm the hum is from the evap fan start/stop. Timing of it is actually more like 1s start/hum, 2s off/quiet. The fan itself continually spins from the momentum of start/stop. Based on that is it the fan motor itself or something in to do with the control electronics (thermostat, timer, etc)?
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Rank: Member
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We've reached the point where internet diagnosis is tough. That's hard to tell without a meter on it. If you have a multimeter on hand, unplug the fan and test the incoming power, 120v AC on those. If it's pulsing then you can assume the fan is fine. If not then there you go. The fan is also powered by the ADC on those, so it is still a suspect. That little board does weird things when it fails. I've recently wasted time at a customer's house looking for a defrost issue in the freezer when it was actually the ADC. How frosted up were your evap coils? A light layer or heavy enough to block airflow through it? Have you had it unplugged for any length of time? Sometimes that resets the ADC and might give you some info. I'd still lay odds on the ADC being the issue, I wouldn't expect that fan to pulse, 120v fans usually fail totally or work. But a multimeter on it should quickly tell you. You could also test it up in the fresh food side at the control, I suspect you'll need to be in there anyway. I think this your diagram: 
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