Does a ceiling fan sound louder when humidity is higher? It seemed to me the ceiling fan is making louder noise(of cutting through the air)  when the day is a bit damp/humid, especially after a rainy day/hour. Is this a valid observation? Can the increased humidity be able to make a noticeable difference in air-cutting noise?
 A: A little long for a comment and I think this is correct.   Ceiling fans make more noise when they are unbalanced.   Any rapidly spinning object like that depends on balance.    There's even a tip that ceiling fans can make more noise and wobble if they aren't kept clean.   See here.
Water vapor in the air can condense on a spinning fan.  Once you have water on the fan blades, even if just a little bit and even with the rapid rotation works to remove the water, you have some imbalance.   A slight weight imbalance = more noise.   You're probably not hearing acoustics from the air, but vibration from the fan.
If there's any dust on the fan, that probably holds water better than a clean fan.    
A: As the humidity of the air increases, its density decreases, so the fan blades will have an easier time passing through the air.  I doubt the fan blades move muchy faster, because they are synched to the motor, but perhaps the decrease in force to pass through the air leads to a louder fan.  What have you observed?
