remove the access door, front bezel, and EMI shield.
Stand up the computer, plug it in, and start it up by pressing the external power button.
As the machine starts up, listen carefully to each of the three fans, and see if you can locate the fan from which the objectionable ticking, whistling, or squealing sound is coming. The CPU fan is the
Can you pinpoint the fan making the sound?
Yes: Replace the noisy fan.
No: If you can hear an objectionable ticking, whistling, or squealing sound, but you cannot identify the source of the sound, contact Apple Technical Support.
Fans are running at a constant high speed
If the fans on the system are running at a constant high speed, or ramp very quickly to high speed and do not vary once this speed is reached, the fans are most likely receiving incorrect thermal input. Follow these steps:
1.Reset the SMC and then test to see if the fans still exhibit the issue.
2.Boot to the EF1 tests of the latest version of Apple Service Diagnostic for iMac. This will test the fans and thermal input of the sensors. If the tests fail, replace the component (s) indicated by the test.
3.Replace the optical drive temperature sensor.
4.Replace the hard drive.
5.Replace the logic board.
iMac
