I require moderately powerful boxes for my work. Moderately powerful boxes generate a non-negligible amount of heat, and therefore require appropriate cooling. Most cooling solutions involve a certain degree of noise, which I dislike.
Water-cooling is supposed to be the most effective solution for noise reduction, but it always felt like too much of a hassle to try to me.
Considering the various solutions I've tried in the past years, two observations I've made really stick out:
Noctua's reputation for CPU cooling is well-earned.
A "silent" (meaning sound-proofed) case, isn't. At least, not anymore.
I've tried numerous coolers in the past, some of monstrous proportions (always thinking that more mass must be better, and reputable brands are equally good), but I was never really satisfied; hence, I was doubtful that trying yet another cooler would make a difference. I'm glad I tried the Noctua NH-D15 anyway. With some tweaking to the fan profile in the BIOS, it's totally inaudible at normal to medium workloads, and just a very gentle hum at full load—subtle enough to disappear in the background.
For the past decade, I've also regularly purchased sound-proofed cases, but this habit appears anachronistic now. Years ago, sound-proofed cases helped contain the noise of a few HDDs. However, all of my boxes now contain NVMe drives (which, to me, are the biggest improvement to computing since CPUs going multi-core).
On the other hand, some of my boxes now contain powerful GPUs used for GPGPU computing, and with the recent higher-end Nvidia and AMD cards all pulling in over 300W, there is a lot of heat to manage. The best way to quickly dump heat is with good airflow. Sound-proofing works against that. Its insulation restricts airflow, which ultimately causes even more noise, as the GPU's fans need to spin at very high RPMs. This is, of course, totally obvious in hindsight.
I removed the front cover from a new GPU box, and the noise went down from almost-hair-dryer to acceptable-whoosh levels. I'm wondering whether a completely open box might not be even more effective.