Persistent Fan Noise very loud on iMac

Using iMac late 2012. 2.7GHz. Mac Os sierra.
Persistent fan noise runs sometimes for an hour. Has been on and off for last hour. Ran Etrecheck and sent report to Apple community. Ran diagnostic and came up with 4MOT…4194. Fans and clean and not blocked. Logic board changed 9 months ago to try to cure but no go. Very disappointed in Apple as I use this for recording audio. Any help welcome.
Jim