I built a custom case with a rotated motherboard like this, with a huge furnace filter on the intake and 8 fans (well in excess of what's needed for cooling) to clean my room air.
Bravo:)
The cheap-o original fans were quiet at low speed but hardly moved any air, and at high speed they made tons of wind noise; I think the airfoils were stalling or something.
The Gentle Typhoons were reasonably wind-noise quiet even at high speed, but made a high-pitched hissing from the bearings at all speeds, and a groaning rumble from the motors at low speed.
The Noctuas are much quieter at 1000 rpm, almost inaudible relative to the 4 hard drives, and make fairly innocuous white noise at high speed.