I thought I had the perfect chassis for my use when this
Sharkoon Rebel 9 Value Edtition came out in 2006, which had one giant 220mm fan moving at only 400-500 rpm blowing in from the side onto the top of the mainboard, also cooling whole RAIDs of 3.5" HDDs in their
vibe fixers (also from Sharkoon) along the way, with air escaping front and back.
Of course, that was before CPUs and GPUs burned through a Kilowatt of collective power.
The basic idea was that if you make a fan big enough, you can make it run so slow, it becomes inaudible.
Somehow I still think that should work better than just going without moving parts all together, yet I've never again encountered a case like this. Perhaps also, because it was really rather economical, more money to be made on bling.
Luckily only one developed a fan noise early on, which I fixed with a spare. But if another should fail, I'll have to replace the entire chassis, because they stopped selling them around 2012.
On my primary workstation/server, which is designed for a compromise between computing plus storage potential yet modest energy use, it's run 24x7 for better than 15 years and kept things cool in combination with top-blowing CPU coolers (also getting difficult to get).
I believe it started with a Phenom II and is now a Zen 9 5950X, and long included both an LSI RAID controller and an Aquantia 10Gbit NIC in addition to a low-middle class GPU, each of which will easily add a minimum of 5 Watts in heat even at idle, as will the chipset and 128GB of RAM. And with 8 SATA devices, there is extra power but also lots of cables not all tucked away from view or air flow: I believe few passive systems are quite as busy and crowded as this.
I added a rear 90mm Noctua fan only months ago to deal with an RTX 5070 replacing a GTX 1060, mostly because it's a narrower and compact 2 slot 2 fan PNY design that's somewhat hot-headed when actually used.
Dust getting into moving parts has long been my other motive for avoiding fans, but again the slower the fan, the less of an issue it seems to be. The steel mesh on top of the chassis' big side fan stops quite a lot of the greater particles, which do settle there and would block it eventually, but it's very visible and accessible so I can just vaccuum them off using a small Dyson I always keep close. The side fan itself stays dust free, even after a long period, but it will blow through smaller particles.
Those collect mainly behind the front grill pieces on the way out, which also sport a fibre mesh behind them. But they are also easy to take off and vacuum without stopping the machine.
It's the faster moving CPU fans which see some pile-up over months of constant operation, but never yet to the point of blocking. It's a dual fan heatsink, just in case, because it's a server after all, and I used to travel quite a bit with a family that had little tolerance for outages.
The GPUs may have profited for the longest time from the fact that their fans stay off, unless they are used for CUDA or gaming, with the big fan providing enough ambient cooling: I never saw significant dust buildup in them.
On the big workstations this single fan chassis no longer works, and I've never found one with an even bigger fan to compensate. There I'm using the typical front-to-back+top setup with nearly all surface space covered via Noctua fans providing flow with as few rpms as possible while avoiding heat buildup. At close to a Kilowatt unnoticeable becomes impossible, but the noise stays relatively "white": the heat wafting from those towers is driving sweat to my face, as perhaps is thinking about the electricity bill.
Unfortunately, most of the software utilites that come with mainboards for things like fan monitoring are plain terrible from a security, privacy and functional perspective, so I won't ever use them. Fortunately HWinfo or similar Linux tools allow at least active monitoring so I can see if heat sports are building up anywhere, while I thoroughly test my systems for stability and errors every time I change them.
But if anyone knows about an affordable chassis, which is a successor in spirit to the Sharkoon Rebel 9 Value with a big slow fan to match a Kilowatt without raising a ruckus, I'd love to hear about it!