Any Noctua brand fan will stun you with its silence. I installed four 20mm fans in a network switch to replace its screaming loud factory fans. They're all running ~1200RPM and it now sounds like a small desk fan. A larger format fan running at a few hundred RPM will likely add very little to the ambient noise around you.
Also big fans (bigger than 140mm) are usually running on noisy ball bearings (or roll bearings) while "standard" 120 and 140 fans are often running on silent magnetic suspension.
