I just did some driving with the rear seat folded down so that I could hear the HV battery fan more easily, because the rear seat hides the fan intake vent. Here are my observations:
During the first minutes of drive the fan didn't start. I drove up to 100 km/h and it still didn't start. Then I floored the accelerator pedal at motorway speed, and I could hear the fan starting and the fan speed increasing. After the motorway the HV battery had warmed up and the fan started at 40 km/h, sometimes at 60 km/h, probably depending on how warm the battery was.
When the fan starts, its speed increases very gradually, at the same time as the car's speed increases, so the noise of the fan blends very well with the road noise. Maybe that's why I haven't noticed the fan before.
but in the driving position I can hear it very well.
I think either you have very good hearing, or your fan is noisier than usual, because I'm still not 100% sure if I heard the fan or if it was just road noise.