I have a 2010 iMac. Recently, the fan has periodically gone very loud (high speed). Any idea why? More importantly, how to stop it?
Try resetting the SMC, which can fix that.
If it doesn't, run the Apple hardware test to see if any of the logic board sensors are failing.
Also check CPU utilization, through the Activity Monitor, when the fans go on high to see if something is hogging the CPU.
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