I used to be able to hear the fan on my 24hr FARS, however... I needed to replace the rechargeable batteries. I charged them up , took down the station and replaced them. While I was at it, I replaced the fan as well and it started up right away off those batteries, quiet.
SO today I am outside checking on the station, garden and thought I would put my ear near where the fan was and couldn't hear a thing. I put my hand down where the screen intake is on the bottom and felt the draft. Man that motor is silent.
Just a note while I am on topic. If and When you have to move the station, when you take off the solar panel from the ISS disconnect the rain plug in , SO when you move the thing, you won't get any false readings. I know about putting the station into setup more if you use a console, but if you use an Envoy, there isn't any setup button and the data has to be manually corrected.