Mine is very quiet, buy YMMV. Can always get a fan-less replacement case to resolve if an issue.
Mine is completely quiet unless it is analyzing new additions to the library.
Hi cheers thanks for the response … I find my 2 core i7 NUC 6 is a bit winy… I use dsp I think that stresses it more. Another question does Roon take advantage of the 4 cores???
For DSP processing, at most 2 core per zone if you enable Parallelize Sigma Delta Modulator.
The remaining cores may be used for other things like UI.
If your aim is to lower fan noise, get a fanless chassis. Upgrade the computer only if you think you need a more powerful CPU for DSP.
OP update: I finally got one. 
NUC8i3, 8GB RAM, 256GB Samsung EVO. 2TB Crucial MX500 SSD for library storage. So far running well, though it’s not as snappy as Roon Core running on my 2017 iMac i5 (started a separate thread for that). I switched the NUC cooling to quiet mode in BIOS and so far the fan only runs at a very low speed during playback, and shuts off at idle. (Caveat: it’s winter and cold here.)
Processing speed seems adequate for single zone with very light DSP use - I do think you should get an i7 if you want to do anything more than that. I’m debating if I should have done that to have a bit more headroom and snappier navigation, but I’m liking how quiet the i3 is - the fan is unobtrusive at low speeds.
Here’s a snapshot playing a DSD file, 4x processing speed. With a 24/96 file it’ll be more like 70x, so it’s really just DSD that taxes it.
If you have engaged quiet mode it may be that you are restricting the available performance, stopping it kicking into Turbo mode.
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