First of all, while you are in BIOS Setup (Visual BIOS), you are running with all power management capabilities of the processor disabled. It is normal to see higher temperatures in this state. Secondly, in the Windows environment, your machine is likely never not doing something; Windows simply has too many background tasks running (my facetious statement: If this is Windows 10 we're talking about, it's the result of all the spyware that Microsoft has running in the O/S!) and you periodically pay a penalty for it.
Saying it as plainly as I can, putting this much performance in a box this size has to cost you something and this is in its thermal management. Because of the small space available to contain it and because of the movement of air that is required over the surfaces of the board (as well as through the processor's heatsink), they were forced to use a blower instead of a fan. Unfortunately, blowers are much less efficient. To move the same amount of air over the heatsink, a blower must run at much higher speeds to do so than a similarly-sized fan.
There are a few things that you can do to compensate for the poorer acoustics. A significant contributor to the irritation of this blower is the pitch of the noise produced. Well, change the pitch! Move the unit further away from where you sit. Put something directly between you and the NUC. Anything. Even something as simple as a pile of books can make a difference. Even better, use the VESA bracket and attach the NUC to the back of a monitor or to the side (or bottom) of your desk.
Thanks to this thread(Re: NUC6i7KYK Skull Canyon NUC runs HOT! ) and the reply of sirozha, finally my brand new NUC is silent on Windows. I cannot even hear the fan any more.
Just changed the settings in BIOS as follows:
Minimum Duty Cycle: 15%
Minimum Temperature: 76C
Duty Cycle Increment: 5%
Processor Power Efficiency Policy: Low Power
Now I am enjoying peace with it!:)
Well, the above configuration seem to made my NUC to go extremely hot in cases where more CPU power was needed.
My current solution is to reset everything to default and proceed with the following SETTINGS in the BIOS:
Low Power Enabled: Enabled
Intel Turbo Boost Technology: Disabled
This has made the fan to constantly spin on 2800RPM and the tempratures to be dropped to around 50 degrees when not playing games/videos.
The drawback is that you are not getting the 3.6GHz speed but only up to 2.6GHz, but at least I have a nearly silent unit.
If anyone can suggest an even better solution to reach even lower RPMs and still have a decent low temperature system, please be my guest and reply on this.