If you only have 1% of diskspace left on any disk it can make the operating system do odd things which will affect the processor even if it is not an OS disk. Free up some disk space by copying things to the external disk and see if it helps.
If you consider a fanless Akasa housing, it is wise to choose the Turing version, because this version provides better cooling material. See: http://www.akasa.com.tw/search.php?seed=A-NUC45-A1B#
A normal hard disk produces heath inside a small housing. It may be prudent to change your internal hard disk for a M2 SSD; I advise to add a heatsink (€ 5,- max) to this M2. SSD; also check compatible versions for your nuc.
And if you do so, RAM memory is not very expensive nowadays. You may also consider expanding your RAM memory to a healthy 16 (or better 32) GB; check what memory is compatible with your NUC.
You don’t mention your operating system. In you use Windows 10, you need sufficient additional disk space for all kind of updates. In general, I would advise you to leave at least 25 % of your disk space free; the same is applicable for the external SSD (or hard drive) drive.
In case you are rebuilding your NUC in another case, I would consider expanding 8 GB to 16 or 32 GB, because very probably your library is very large. Besides RAM memory is not expensive nowadays, better be on the safe side with RAM. See also the next link: Best hardware for Core for large library
You had not mentioned your OS. Windows 10 needs unspecified additional space for upgrade etc. Now you write that you use Roon ROCK and taking into account that you use a 2 TB M2 SSD, you probably can leave far less than 25 % free space, but having only 1 % free space (or 20 GB) as free space may cause problems in case you will accidentally and unwanted copy several albums to the disk that is nearly full.
I don’t know if you use DSP upsamplingc: upsampling to DSD 512 is very processor intensive. My NUC in Akasa housing remains below 45 degrees Celsius when I upsample up to DSD 128. Above the housing gets lukewarm (DSD 256) or rather warm (DSD 512).
How big is your library by album count? I’m guessing around 12k. Using ROCK on that NUC, 8GB of ram is good enough to probably ~80k-100k albums – I wouldn’t spend the time or $ to upgrade RAM beyond 8GB.
Huge… as you see by the 90%+ free. I dont know if you have a SATA or NVMe drive… but NVMe will perform better, but it wont help with your current heat/fan dilemma (it shouldn’t hurt it either).
It lies to you so you have a buffer. This is typical of all file systems. Kinda like the gas gauge in your car.
lots… first is audio analysis… how long have you had this library on this install? the next is periodic metadata updates… they can take hours, but usually not days if your hardware setup is good (which it seems to be).
A dusty FAN will cause temps to rise and fan to run faster but if its dusty and clogged its just going to get worse until it thermal protects. Remove the fan and air dust (canned air) it out then see how that goes. also blow out the heatsink fin channels
Oh i meant 120k+ album count so i will upgrade RAM next time.
The M2SSD is an 256GB ADATA XPG SX8200 Pro NVMe drive.
I moved some files to the external drive, roon says now 10% free of 1848GB.
I will give the the fan a look, the NUC is running a full year without maintance.
Dust is allways a problem.
And i am sure i found the main problem, roon was running audio analysis.
I tried to change the drives and copied and added files and deleted the whole database.
For testing i switched off the analysis and the NUC was quiet immediately.