It is certainly not "whining motors and bearings" that makes them loud - it is the fact that you are creating massive periodic pressure differences with the blades. It is the same reason helicopters go thump-thump-thump, but scaled up in frequency. Have you ever heard a quiet propeller? About the best you can do is increase the size and decrease the speed (more efficient = less power dumped into the air), add more blades (minimizes circumferential pressure variation), and add a duct (eliminates tip vortices). But eventually you hit weight limits on all of that. At drone sizes, unless the drone is impossibly feather-light, you're always going to have some sort of whine.

Even in the absolute theoretical best case scenario of an idealized disk-shaped actuator inside a duct with proper lip shaping and 0 turbulence around the actuator, the shear boundary of the high velocity output plume is unavoidably going to cause turbulence. The best you can do is "woosh".