i agree it's a good question. First, can you swap the two and see what noises the two make then? If the noise goes with the dimmer, it's the dimmer. Why? Well, we'll get to that, umm, OK?
If the noise goes stays with the circuit and fan, it's that. (duh). Then, it would either be the fan or the circuit (duh0).
After all that, i'll be glad to add other duhhmb remarks guessing about possibilities. I also like to mention inductive loads and power factors. so give me a chance to say it please.
David