Why is my microwave making this weird noise?
May 4, 2021 10:59 AM   Subscribe

My microwave is making a weird noise, when it is plugged in but not running. I have taken to unplugging it when it's not in use, but I am very curious what it is, and if it's something to be worried about. Weird noises are notoriously difficult to describe, especially over the internet. But I will attempt to do so within.

The first word that comes to mind is "crackling," but that implies that it sounds like either fire, or electrical arcing, and it does not resemble either of those. Also there is no other sign of electrical arcing (smell, sparks, malfunction, etc.). I might also say something like water dripping, but it's not quite like that either. A cross between water dripping and crackling? It's something that I probably wouldn't be surprised to hear from a freezer or refrigerator, but it seems strange for a microwave. It is intermittent, not continuous, but happens every few seconds I think. I haven't timed it, but I can say that I almost always notice quickly if I have forgotten to unplug the microwave. It's not loud, but it's audible enough that I can hear it sitting on the couch 10-15 feet away.

I have googled this in every way I can think to describe it and have come up short. Most of the "weird microwave noises" that turn up are things that happen while it is running, but I hear this one when it is not running. I can't say for sure if it happens while the microwave is running or not. I haven't noticed it, but the other noises may just be drowning it out.

Any ideas what is causing this, and if so, is it harmful? Should I dispose of this microwave, or is it ok to keep using?
posted by primethyme to Home & Garden (7 answers total)
 
I have googled this in every way I can think to describe it and have come up short. Most of the "weird microwave noises" that turn up are things that happen while it is running, but I hear this one when it is not running.

Did you search “microwave makes weird noise when not running”? I just tried that and everything seems to say it’s a problem with the magnetron, which, I don’t actually know what that is but it’s a place to start!
posted by showbiz_liz at 11:08 AM on May 4


Most microwaves have a switching power supply incorporated into the control board. That might be in the early stages of failing and is making a periodic ticking noise. That's the only part of a microwave that's active by design when it's in standby. I'm not going to speculate in writing about the safety of that condition.
posted by Larry David Syndrome at 11:23 AM on May 4 [1 favorite]


Any chance you could grab a recording of the noise with your phone and stick it online somewhere?
posted by flabdablet at 11:57 AM on May 4


Does it sound anything like the coil whine that this guy is showing us how to get rid of by replacing the whining coils?

If it does, and it's changing in step with changes on the front panel display (does your oven have a time of day clock?) it's probably harmless. Coil whine is pretty common in the shitty coils they put in shitty mass-produced switching power supplies, it's sensitive to the amount of power being drawn (a glowing seven-segment display showing an 8 draws more current than one showing a 1, for example) and it sometimes does begin to manifest only after the supply has been in use for a few years.
posted by flabdablet at 12:24 PM on May 4


The crackle sounds like a high voltage discharge, and microwaves do contain a high voltage capacitor.

But if it's on standby, as Larry David Syndrome suggests, the capacitor is probably meant to remain fully charged, so I'd guess the drip sound is a relay which trips when the capacitor is discharged to certain point in order to charge it back up again.

But that should happen less frequently, so it seems likely that something is wrong with the capacitor circuit, perhaps just some kind of debris near the capacitor contacts that is allowing it to discharge.
posted by jamjam at 1:07 PM on May 4


More likely a bad capacitor though, which might need to be replaced.
posted by jamjam at 1:11 PM on May 4


if it's on standby, as Larry David Syndrome suggests, the capacitor is probably meant to remain fully charged

That makes no sense to me. The high-voltage capacitor in a microwave oven is part of the circuit that provides the magnetron, the component that makes the actual microwave radiation, with the high voltage it needs to do that. I can think of no reason why that capacitor should need to retain charge while the oven isn't actively making microwaves.

In fact, good design practice for any such HV capacitor generally involves wiring a very-high-resistance bleed resistor across it specifically to discharge it slowly when the oven isn't in use, lest it unexpectedly retain charge and Electroboom an unwary service technician. I've never seen a microwave oven HV capacitor that doesn't have a bleed resistor associated with it.

The only capacitor in a microwave oven that should have relatively high voltages present when the oven is in standby will be on the input side of the switching power supply for the control circuitry and display panel, if that's what the design uses for that (many just use a small conventional mains transformer instead). The highest voltage present across a switching power supply capacitor won't be much higher than mains voltage, though, and is therefore unlikely to be responsible for making an audible arc, especially not one that isn't obviously letting any of the magic smoke escape.

The most common source of audible noise from switching power supplies is coil whine from inductors, and that's harmless though irritating.
posted by flabdablet at 9:29 PM on May 4 [1 favorite]


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