Mine is in the kitchen for exactly that reason. No matter how quiet it is, any sort of constant noise like that of an hdd drives me nuts. But I fully agree normal desktop tower is the way to go. Enough space inside so it doesn't easily overheat and lots of room for expansion.

Using it for routing with DNS block list, media file server and off site backup.

One thing I can recommend after decades of fiddling around with home servers: If you don't need it for the high availability, don't use raid 1. Raid is not a backup. Use another drive, preferably larger, and run something like dirvish or Borg once a day or however often you see fit in your case.

I have a Fractal Design Define R5 case (https://www.fractal-design.com/home/product/cases/define-ser...) in my living room at the side of my television where I sit a few feet away in a small apartment and can not hear it unless I put my head right next to it, then I can just about hear the case fans.

It's fitted with 4 x 4TB WD Reds, a stock AMD Ryzen 2600 cooler, Corsair RM850x PSU and the cheapest graphics card I could find with a HDMI out, the graphic card has a small fan. I also have a HBA card I modded adding a small Noctua fan as they are passively cooled and designed to go in rack mount cases where they have high airflow passing over them so get to hot in a desktop.

The case has rubber mountings for the hard drives and sound reducing material on the side panels.

I built it to be quiet but didn't go massively out my way or spend massively to make it quieter i.e. I choose a good case but have stock case fans, stock cpu cooler and choose a PSU where the fan only spins up under load which I'll never hit.

It's not to hard to build something that for all intent and purposes is silent as long as you don't pimp it out with the latest fastest graphic cards, have it maxed out 24/7, or go small.

May be someone should be a sound proof case for a JBOD..

Disks suspended with cables? Box filled with sound absorption materials? Whatever it takes.

Or if that cost ~= cost of SSDs, just buy SSDs.

Me? I have a 12 TB SAS disk connected to the system with an easy to turn off switch so I power it up only when I need to backup something. Others times it stays unmounted and powered off. My whole digital footprint is only a TB....

You can solve HDD noise issue in a tower case DIY style by suspending HDDs in some rubberry material. There is plenty of space for that where CDROMs used to be put. Or buy a tower case that has anti vibration for HDDs already.

You are right, 3.5" disks can be loud, 2.5" drives or SSDs are better in that respect. Other option is to spin down / suspend the box after inactivity.