[afry1]

Those Lasko fans have pretty raw edges on the blades of the fan itself, which I think contributes a lot to the noise. If you take the cover off, sand down the nubs and bits of flaking plastic, and reassemble, I think that will take care of a lot of noise.

[BizarroLand]

Blade geometry would make a lot of difference. Could be a fun at home science experiment to do with a 3D printer and $200 worth of filament.

[IAmBroom]

You want to discourage turbulence at the tips. Blade tips, like the risers at the edge of some jet wings, would probably help.