Originally Posted by scirel
That's what I was thinking. I'll be the first to admit that I don't know how aircraft pressurization systems work, but I do know that in my car when I switch to outside air from recirculated air the noise gets softer. The recirulated air is actually louder.
That's because you're now hearing the suction of the air being recirculated back into the system. If you notice, you probably hear the new louder sound coming from underneath the dash where the return air goes back. When it's coming from the outside, the intake (and the associated noise) is away from the cockpit of the car. The noise at the outlets (cold or warm air) stays the same.