iMac27 - overnight fan noise much louder

I turn my iMac off each night. This morning I find that the fan noise is very loud. The exit air is nice and cool, activity Monitor does not show excess activity. The room is not warm.
Anybody have any ideas? Thanks, in advance.
TR

Fan Noise on 27 Inch iMac

Same thing happened to my 27" iMac, This fault happened 1 day short of 6 months after purchase date, So I could still Purchase Apple Care. Then I called Apple and Apple sent a Tech out to the house & replaced the Mother Board on my computer. (house calls are part of Apple Care if you can't bring to a Store). Computer was too heavy and bulky for this old man. This was done incorrectly by the Apple Tech. So now the computer had more problems. I then got some help and had to take the computer to a Apple Store and the Genius replaced the Computer at No Charge. This took 1 hour......And I walked out with a current 27" new Model. You can't beat having Apple Care. I will never but another computer without it.

The Apple free replacement program only applies to the 27" iMac models which have the Seagate 1 TB hard drives installed and then it's limited to certain year models only. It does not effect the 21.5" models.

I thought we were talking about the Fan noise, not the HD replacement program on 1 TB Seagate HD.
The fan going to high speed on my 27" iMac was a failure on the Mother Board that happened 6 months before Apple determined they had a Hard Disk problem.
I would think if this happened on any mac under Apple Care Apple, Apple would take care of it.