Have you replaced windshield or had it worked on lately? Sounds like maybe when glue or windshield trim gets cold it might pull away from vehicle causing wind sound. Also could happen the faster you travel. Does this happen when it is really windy outside as well?? The windshield is an easy and cheap place to start in addressing this issue.
If this is not it then I suggest getting an air compressor and an air-blowing tip and having someone blow air into the front of your car while you sit on the inside to see if you can figure out where it is coming from. You could also call "Car Talk" on Saturday, those guys are awesome and love questions like this. www.npr.org