Chosen Solution
Apple used proprietary hard drives on this machine with 7 lead temperature sensors and does not sell these to the public. Causing the fan problems. his is a very new machine for the hard drive to be going out. Do you still have it?
When I took apart my iMac (probably nearly the same generation; I have an 11,1) the first time, I messed up the vsync cable, which caused some strange visual and audible problems (high pitched whining). I would just go back and check the cables you unplugged to get under the display – it’s hard to make sure they’re all snuggly attached, since you can only lift the display a few inches. Use a flashlight to make sure that all the cables are very secure. Make sure that the vsync cable (the small ribbon) is attached on both sides, as it just slides out of the other side easily. Good luck! iMacs are great machines :)
