I love Noctua, and just wish they'd branch out from PCs and build more types of fans. Our lives are filled with so much irritating noise from noisy fans. Air conditioners, kitchen extractors, hair dryers, box fans, air purifiers, vacuum cleaners, leaf blowers, car climate control & radiator fans, just to name a few. I'd happily pay a premium for quieter things.
As far as air purifiers are concerned, there are well-regarded designs you can buy that use PC fans (you can order without fans and put in your choice of fans) and furnace filters. I just got a North Box Polaris: https://aidankepo.wixsite.com/northboxsystems. Luggable supplies an equivalent for Americans.
I don't own a (stationary) PC, but I have bought Noctua for other projects due to them having good reviews. Was surprisingly hard to find good fans for my usecase, that wasn't industrial (pricey). And PC fans are easy to control.
I'm just expecting a reduction in volume, vibration, and unpleasant resonance.
In any of these categories you can already find some machines are quieter than others despite comparable air flows. So its definitely possible to reduce noise through clever engineering and precision manufacturing.
