I have a late 2014 5k iMac with 256GB SSD drive. Last night my iMac started making noises similar to that of a disk drive when it's reading and writing.
After poking around I found that the noise must be related to the GPU because:
- The noises increase when scrolling a webpage or change workspaces and then subside when I stop.
- Running a web GPU benchmark or watching a video will make the noise constant until I close it
- Also changing the brightness causes the noise
I doesn't sound like a bad fan as it's a very clicky noise versus a whine or hum.
I ran the hardware diagnostic test (Holding D when you restart) which resulted in no errors. I tried to blow some air in the GPU vents which didn't change anything.
I feel if I take the iMac in and describe the issue they will send the iMac away for few weeks and it will be sent back saying there isn't any issues.
Here is what it sounds like. It also makes this sound when the display is off, so it could be like @Allan said that it is the power supply.