iMac humming noise

[superdudeo]

Hi guys,

Would appreciate any help on this. My 2010 iMac had it's optical drive replaced with an SSD about a year ago. I have recently noticed a humming noise that I cannot get rid of. I have tried playing with the fans with 'SSD Fan Control' with no joy. I am assuming it's the internal HDD because when I put the iMac to sleep, the noise goes.

Can anyone help get rid of this incessant noise? I am running El Capitan. I have tried a SMC reset but still no joy.

 

[superdudeo]

So i followed the advice in this thread and realised the humming goes away when lifting the iMac! Only thing is, I have put soft padding underneath the iMac but the humming still there. How strange.

http://www.mac-forums.com/forums/apple-desktops/224581-27-imac-hum-resolved.html

 

[ApfelKuchen]

Nothing strange about the padding not being a help. The physical wavelength of a low frequency tone is much greater than the physical thickness of the padding - that's why you can hear the thumping of bass notes through a wall, even when you can't hear the rest of the music.

One thing that kind of padding can do is de-couple the Mac from the surface, so that the transmission of vibration is less efficient. That, however, assumes the Mac is what's vibrating (or resonating) and even that is not certain. The weight of the Mac may be putting the desk in stronger mechanical contact with the floor, and it's the building and desk, not the Mac, that are vibrating.

A desktop can easily function as a resonator, acoustically amplifying a relatively small vibration. The placement of the iMac may be affecting a resonance "node" in the surface it sits upon (like a finger pressing a string against a fingerboard) - a different desktop with different dimensions and/or different stiffness, or placing the weight in a different location on that desktop may change things.

One thing I was going to note before your most recent post is that it was premature to conclude that the HDD or fans are the probable cause. I was going to suggest plugging a set of headphones into the Mac, and/or adjusting the speaker volume. Also, what would happen if you set System Preferences > Energy Saver so that the display went to sleep, but the computer did not. All these may still provide some hints as to cause as well, though they seem less likely based on your second post.