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Model A1418 / Late 2013 / 2.7 & 2.9 GHz Core i5 or 3.1 GHz Core i7 Processor

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Where is the source of buzzing noise?

I just bought a second hand iMac and it works well in the first several days. Then there is some buzzing noise (the frequency varies, but usually pretty high) when I run some computation scripts (100% CPU usage, the frequency of noise is high) and when I scroll the webpage(the frequency changes compared to doing nothings, but not as high as running scripts). The amplitude is not so high but it’s noticeable. I asked the seller and he claims he never heard that.

I doubt the source of the noise might be power supply or CPU. Since the noise is still there when I locked the screen as long as there is some tasks running. And, I also launched the iMac into Target mode, under which the noise still exists and the frequency increases. To exclude other assumptions, I also used a UPS for all above tests.

I searched the web and found many people had this same issue with various modes of iMac (from 2008-2017). But most of discussion didn’t lead to a proper result. So my question is how to locate the exact source of this noise? And would it cause some damages to the board if I keep ignoring it?

Here is a link to one sample of the noise when I was running some code,

Thanks in advance!

Update (03/24/2019)

The iMac just let out a LOUD POP sound and really scared me…, in this quiet evening. There is no obvious smoke or smell and the iMac runs as usual. But I really doubt this is caused by a capacitor explosion in power supply. Just wonder could it be possible for the machine running with an exploded capacitor in power supply? Thanks!

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That’s called inductor whine, it is more or less normal unless a known quiet computer suddenly starts whining under the same workload.

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Thanks! Usually it just buzz slightly with some random intervals, though I'm pretty sure it will increase with the power consumption increase. If this is the case- it's inductor whine, is there anyway to solve it by replacing some components?

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The bigger inductors are within the power supply. You could try replacing it.

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The recording is very hard to hear the whine. Many audio systems cut off these higher frequencies and age can also limit ones hearing (as in my case). @tomchai can you hear it?

Besides coil whine that Tom talked about …

  • Fan bearings can whine when grit gets in, as the fan ramp up or down it will change as the CPU & GPU workload changes. Using a fan control app like TG Pro should be able to help you see whats happening and you can also push the fan up to see if that was the issue.
  • Switching transistors (MOSFET’s) and electrolytic capacitors can also whine at the AC line frequency.
  • Your amplifier and speaker maybe picking up a bad filtering capacitor. This is the hardest to locate as there are just so many. See if a sound is present within a plugged in headphone or external speaker and also try a Bluetooth set as well.
  • You could also have a loose screw or cable which is banging around.

Do make sure your system is properly grounded all the way back to your breaker panel and the ground service from the panel is good as well. Often the ground line is tied to a water pipe exiting the building or a grounding rod is used. Corrosion buildup on the clamp and the wire mount can mess you up. A rare occurrence can be corrosion causing resonance feedback from the neutral line and ground lines.

Given the systems age I would give the system a good cleaning. Do keep in mind the newer ‘Thin Series’ iMac’s are harder to open as the display is glued to the case! If you don’t use the correct tools you can damage the display. Here’s the needed tools

I also find using a suction cup helps in holding the display helpful:

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Thanks for your suggestions! I've tested the fan via MacFanControl and indeed the noise frequency increased a little. However, it is nearly zero increase compared with running scripts. Also I connected the headphone to the mac and, the noise still exists. Since the noise is steady when entering into target disk mode, I think it could be related some hardware issues. But as you said, it can also be a problem of ground line. But how to check the status of the ground line, usually they are hiding in the wall?

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As I stated I think a good cleaning of the dust build up might improve things. Dust act as a blanket holding the heat in so when a component runs hot it can resonate.

A good DVM or using an oscilloscope should help in locating the source within the system.

Getting an electrician in to review your power outlets and the buildings power & ground services making sure everything is wired correctly.

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Im just thinking the noise would be like a clicking sound, i suspecting your hard dive because its still a spinning drive and when you run too much task it makes a noise a lot than using it normally

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I don't know what exactly a clicking noise is... Here is another sample recorded when I was scrolling the web page:

https://www.dropbox.com/s/rvdn1kklcqyc8y...

Can you help me identify the type of the noise? Thanks!

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im not sure if it was the fan because you said when your running a lot of heavy application the sound getting noisy.

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