Yeah, as others have stated, it's just a consequence of the car being more aerodynamic, which is usually a good thing!
Since the car is very aerodynamic, air wants to flow very close to the back windows smoothly and at high speeds. When you open the windows, the fast moving air wants to suck the air inside the car out (think airplane wings). That create lower pressure inside the car which in turns wants to suck outside air back in. This goes back and forth quickly causing the noise you hear. Cracking a front window open, even just a little bit, causes the internal pressure to maintain equilibrium more easily, disrupting the high-low pressure cycle. Same goes for cranking up the fan in fresh air mode. This pumps air in the car causing positive pressure which helps prevent the air being sucked back in.
This also happens with the moon roof. Open it on the highway and you'll hear a constant hiss noise. That is the sound of turbulent flow caused by the moon roof deflector. If you lower the deflector with your hand you cause the air flow to become smooth, fast and close to the opening (as MikeM said, this is called laminar flow). The noise will instantly become "booming" in nature, caused be fast alternating low and high pressures in the car.
This geek moment was brought to you by physics, we now return to you regular scheduled programming...