Hello,
A few days ago I cleaned the air hole above my furnace. This hole sucks stinky kitchen air out of the kitchen and blows it up into the sky through tubes.
I noticed how this hole started to hum after it was cleaned.
Apperently the fluffyness that was on it, either reduced the airflow, or it simply distorted waves of sound.
So my theory/hypothesis is now basically very simple:
Even surfaces will lead to oscilliation/resonance of sound waves.
The smoother/even a surface is the more sound it will produce.
By adding fluffyness or perhaps other bums/imperfection it will break the oscilliation and perhaps lead to more distortion/barely noticeable sound.
Or perhaps the fluffyness itself is responsible for petting the sound waves.
It's probably as soft as a feather/hairs... it's composition is probably human dust and some oil/benzine burned particles from cars and busses.
Not sure if some special grease must be used for the fluffyness.
But it's worth a shot to apply to fans and see if this will actually make them more quite, or more loud.
I do know if fans get stuff in their shaft that it will produce terribly sounds... but maybe if it's on the blades it actually reduces sound... my expectation would be that it increases sound... but perhaps this is wrong. Or perhaps it depends on the pattern of fluffy.
The fluff looked like little hairs, with air gaps between them... as if it was stroking the air gently ! ;) =D