Update: I went into the bios>cooling menu and reduced the "minimum duty cycle" from 35% to 25% and all the fan noise is gone. Nuc is working as before; no change in performance. Learned about this trick from a review on Amazon for a broadwell nuc. FYI, I use the nuc for work, emails, netflix etc. Nothing more intensive than that. Do you see any problems with his tweak or have any other suggestions? Thanks a lot.
That should not be a problem, you could also check the temps your NUC is having while using this BIOS setting.
Third party software would be useful for this, personally I use Speccy and does it's job providing you with the temperatures of your components.
Here the link to it's website: Speccy - System Information - Free Download
I set minimum duty cycle to 25%,
When I browser internet with Chrome, the CPU temp is around 50C,
When I watch a high resolution movie, it goes up to 60C.
Play game, 70-80C.
Is that a normal temp?
What is the acceptable temp range for NUC6I5?
These are all perfectly normal temperatures to see in these conditions. As I stated in my guidance (see above link), your processor's Tcontrol temperature is somewhere in the vicinity of 80C. So long as you are using a Rise value that gets you to 100% duty cycle at or below 80C, you should be ok.
Indeed those temps are fine for the usage in the unit.
I would like to recommend you to check the specs of the CPU inside your unit, please refer to "TJunction" under "Package Specifications".
Any temperatures below that are ok and the unit would work without harm.