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My late 2012 has been making this annoying rattle sound for months now, on and off but most of the time it makes the sound. Sounds like a helicopter hovering over my house. I will call Applecare and see if they can send a tech to my house. Because I work with computers and know that if I bring it to their store the damn noise will stop and wait to start again until the machine is back at my house.

Because this model only has a single fan I know it must be that one. Or the hard drive (I have 1TB fusion so it has a disk that spins in there) but it goes up and down in loudness which makes me think it's the fan and changing speed, the hard drive would likely be more constant and perhaps even stop if the disk sleeps while only using SSD data.
 
I called Apple and they sent a tech to my house, replaced the fan, rattle is gone :)

So good to have a quiet iMac again.
 
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