iMac: Fans run at full speed after computer turns on
After plugging in your iMac and turning it on, the fans may be noisy and appear to run at full speed. In this event, shutting the iMac off and turning it on again may not resolve the issue.
If you press the iMac power button while you are connecting the power cord, the iMac will enter a mode in which the fans run at full speed. In order to correct the issue, reset the SMC. Your iMac should now start up and operate as expected.
To avoid this issue, be sure to start the computer by pressing the power button after you fully connect the power cord.