Battery Fan Running a Little While Driving?

Sometimes when I'm driving my P.Prime in EV mode, which is pretty much synonymous with when I'm driving my P.Prime, I turn my head to the right, and I think I might be hearing a very-quiet hiss from the back seat area.

I'm debating whether that could be my imagination, the battery-cooling fan running during discharge (i.e., not just during charging), or a higher-frequency component of road-surface noise coming from the back tires.

It's hard to be sure whether or not it goes away when I'm at a stop, but I think not. Then again, even if so, it's not inconceivable that the cooling fans might also be turned off when they're not driving the motors.

What do you folks hear along those lines?