They are louder than proper cars, but generally the noise is masked to a great extent by other noise (also louder than proper cars) as you are running along.
Have you got a rumble (bearings etc) or is it higher frequency fan blade noise?
I guess you will find what I found, the fan is white plastic and the shroud is black plastic. My fan was some black plastic melted to it from the shroud where it had been touching.
I had tried oiling the bearing but that only lasted a few months. This time I tried greasing it - lasted a bit longer but has started again and slowed the fan down