I use Intel NUC. It is small, quiet and you can choose some components as you like. For a homelab, be sure to have a CPU that supports virtualization (VT-x), so you can play with VMs. On top of that Proxmox.
Like someone already mentioned /r/homelab for more ideas.
Many comments on Reddit and such complain about the noise made by their NUC, and I hate my NUC8i3BEH because of the noise the cooling fan makes. I think some people are sensitive to constant sounds of constant frequency and some are not. I.e., for me it is not about decibels, but more about spikes in the sound spectrum at a certain frequency that persists as long as the computer is powered on.
This isn’t the first comment I’ve read about the noise, but I’ve had an 8i5beh for almost 5 years (just replaced a dead fan last week!) and a 13anhi5. I never notice them, they’re essentially fanless to me.
Currently sits at 40db in my bedroom with the fan at 100% 24/7.
Is it a high pitched whine? Or is it like… unfavorable harmonics? I have an old brocade switched that had a very discordant sound, I did end up replacing those fans.
If constant 40db noise doesn't bother you, then you might actually want to see an audiologist... It's possible your hearing is much weaker at some frequencies. Treat yourself to a proper hearing test, if nothing else just to satisfy your curiosity.
Whines and drones (whines of lower frequency) are what I hate if they're present all the time or most of the time. It's not discordantness that bugs me.
I feel you. I was very disappointed with the noise made by my nuc7i5bnh. Did a bit of research and found out that a) the default BIOS fan settings are crap and b) the installed fan is crap as well.
Found some recommendations for the fan [1] and BIOS fan settings [2] on the internet and got my NUC completely silent. Perhaps that works for you as well?
I can recommend going for fanless cases, like the one from Akasa, Streacom or Impactics. In my case I am using several Akasa Max MT and Impactics D2NU1 (D2NU2) cases and they are perfect for my all-silent home lab.
Sadly the turemetal cases are not available anymore :(
Like someone already mentioned /r/homelab for more ideas.