My iMac 27″ 2011 started hissing at me a couple of weeks ago.
You especially notice the hissing when the screen is at it’s brightest.
Listening to the back of the machine, you can hear the hissing at the top left-hand corner (when looking at the screen).
This is a sign of a bad power supply, and it needs changing.
When power supplies make this noise, it means something in the unit is not working correctly, and this means that it’s not providing the right power, and eventually, something else will give.
Luckily, with my trusty “Mac” toolkit, a “new” part from Amazon for £67.99 and a quick Youtube clip, I was ready
The whole process took less than 30mins to do (plus around the same in prep and watching a Youtube clip)
As you need to take the screen out, it was a good opportunity to blast those fans with air. (Don’t do this with the screen in, that would be stupid!! as you’d get dust between the glass and screen!)
Now my iMac is super silent again and hopefully good for another couple of years!!