I agree with baddriver about what makes the noise- things sticking out disturbing the wind, like the side windows not being (flush) with the door frame, etc. I bet that the wind going through the front grill, radiator, then down through the engine bay makes noise. Don't forget about the drive shafts (3 of those).
BUT, I suggest you keep in mind that you don't want to make the car so quiet that you can't hear traffic beside you. I'm talking about somebody tooting the horn AT you or FOR you, or an emergency vehicle's siren approaching, maybe even a cop directing traffic. For the same reason I don't care for loud radios- you can't hear what's outside.
my 2 sense.