Big fans are a good thing because they move much more air with much less RPM. If you care about noise, buy components with low power requirements and low heat output and slam a ridiculously overpowered cooling solution on it. I recently tried a workstation radeon GPU with one very small fan that would be essentially silent if it were in a case. Something like a recent i3 or i5 would mean so low heat output that high end coolers from Noctua would probably work somewhere around the lowest RPM with correctly configured fan curves. That, combined with the possibility of putting the case below the desk instead of right next to yourself like a laptop sounds much more silent as my macbook pro. An efficiently cooled i5 should be much more powerful than the frequently thermally throttled ultrabook/mobile CPUs. Those components are usually the cheaper ones too.

Granted, my high end gaming machine can get a bit noisier compared to my macbook, but even then the kind of noise is more pleasant than the relatively high pitched wind coming out of the MBP. The 3900X and the GPU do generate a ton of great though, so the focus is not on noise and heat output like in your case.

Intel’s line of mini PCs have worked well for me. They call them the Next Unit of Computing, or NUC.

Older generations [of the hardware!] have more stable Linux support. I currently have a gen 4 Intel NUC (Haswell) and it is a lovely platform.

Not a lot of local compute power though. For that, it seems better to go all-in on well cooled datacentre hardware and connect to it remotely.

I installed liquid cooling + big fans which often don't rotate fast (depending on the load).