Crackling noise - 27" 2014 imac

[SamWijib]

Hi All,
My first post on this forum and it looks like this might be a problem that's been discussed before but I can't find anything that exactly matches my symptoms. There's something inside my Imac that intermittently makes a low level crackling noise.

The imac was bought second hand before Christmas. The seller said it had a persistent crackling sound (so it's not a new thing) and when I asked for details he said that it had been happening after the fusion drive was replaced by a 2TB SSD (BX500) and it wasn't on the headphones. When he checked further he said the noise had stopped. I've been very happy with it since I got it, and as he said there were no noises, I installed Monterey with OCLP as soon as I got it and it's been great - just browsing, office stuff, zoom - nothing taxing. But the crackling noise just recently came back. It sounds like it's coming from the vent at the back - but where else would an internal sound come out? so not necessarily located just there. It initially sounded to me like worn ball bearings clicking or fuzzy electrical arcing (very rapid but random ticks), but I just noticed that it interferes with my stereo, the amp is near to the imac and the same sound comes out of my speakers, so I'm concluding that it is electrical arcing. However there is no smell I can perceive, not burning or ozone. The sound is not always present, the imac has been on now for 3 hours and it's completely silent and sometimes it can go for a full day without noise. When the noise is there I've tried controlling the fan with "Macs Fan Control" and speeding it up makes no difference to the noise - the crackling doesn't become more rapid - I thought it was maybe connected to the electrical activity of the fan, if not the bearing. The noise does go away when I put the Imac in Sleep mode and comes back when it wakes up - which seems to indicate that it is connected to the fan activity (?). Sometimes if I shut it down and restart after a short time the noise is gone, other times it's still there. When the noise is there it's continuous, not coming in bursts or off / on.

I can definitely live with the noise as it is low level and is drowned out by regular level youtube videos and it's not on the headphone out. But I'd like to get to the cause of it and fix it, in case it's slowly causing damage. So if anybody has any suggestions what it could be it would be most appreciated. I can't think of any other moving parts apart from the fan - the old HDD was replaced by a budget SSD before I got it. Also, is there a way to stop the fan completely for test purposes? As far as I can tell, Macs Fan Control doesn't allow for switching it off or setting it below 1200. Is there an easy way to stress / warm up the Mac to see if it's heat related? - it doesn't seem to be as the noise can start when the CPU cores are cool (below 40C).
Any ideas?

 

[vishvajit]

It's possible that the issue is related to the power supply, as electrical arcing can cause a crackling sound. In this case, the noise may be related to the power supply's voltage regulation or capacitors or maybe they have partially burned and needs replacement.

To further diagnose the issue, you may want to try the following steps:

1. Check for any loose connections: Make sure all cables and connections inside the iMac are securely connected.
2. Monitor the temperature: Use a tool like iStat Menus to monitor the temperature of your iMac's components. If you notice any unusual spikes in temperature.
3. Disconnect external devices: If you have any external devices connected to your iMac, try disconnecting them to see if the noise goes away.
4. Boot in Safe Mode: Boot your iMac in Safe Mode (hold down the Shift key while booting up) to see if the noise goes away. If it does, this could indicate a software issue.
5. Have a professional diagnose the issue: If the issue persists after trying the above steps, you may want to take your iMac to an Apple Authorized Service Provider or an experienced repair shop for further diagnosis.

It's also worth noting that disabling the fan completely is not recommended, as the fan is necessary for cooling the iMac and preventing damage to its components. If the noise is indeed related to the fan, you may need to replace the fan or have it repaired.

 

[SamWijib]

It's possible that the issue is related to the power supply, as electrical arcing can cause a crackling sound. In this case, the noise may be related to the power supply's voltage regulation or capacitors or maybe they have partially burned and needs replacement.

To further diagnose the issue, you may want to try the following steps:

1. Check for any loose connections: Make sure all cables and connections inside the iMac are securely connected.
2. Monitor the temperature: Use a tool like iStat Menus to monitor the temperature of your iMac's components. If you notice any unusual spikes in temperature.
3. Disconnect external devices: If you have any external devices connected to your iMac, try disconnecting them to see if the noise goes away.
4. Boot in Safe Mode: Boot your iMac in Safe Mode (hold down the Shift key while booting up) to see if the noise goes away. If it does, this could indicate a software issue.
5. Have a professional diagnose the issue: If the issue persists after trying the above steps, you may want to take your iMac to an Apple Authorized Service Provider or an experienced repair shop for further diagnosis.

It's also worth noting that disabling the fan completely is not recommended, as the fan is necessary for cooling the iMac and preventing damage to its components. If the noise is indeed related to the fan, you may need to replace the fan or have it repaired.

Thanks for the suggestions - it's some sort of arcing for sure, the electrical interference in my hifi points to that, if it was some sort of knocking or worn bearings that shouldn't interfere with the hifi. Although it's not recommended, I'd like to experimentally stop the fan the next time the noise is there, if the noise stops with the fan, then that would indicate the fan being the source. The noise stops when I put it into sleep mode, which stops the fan - but what else does it stop? The screen maybe? Could be the power supply if that goes into a lower power mode when it's in sleep. I'd check the connectors but opening it up is a pain. When the noise starts, there doesn't seem to be any cause for it - no movement of the computer triggers it or stops it, so I'm doubtful about a loose connector - why would it start and then fix itself and then start a few days later. It's not happened for a few days - next time it does, Im going to lower the screen brightness and see if that makes a difference. It would also be handy to be able to stop the fan, without going in to sleep mode if you know how it can be done. Is there a terminal command?