Here is a video I took. Underneath the wind noise you'll hear a soft humming or droning, that is the sound. My girlfriend was holding the phone (and not with the microphone in the right direction) which is why I don't think it's very loud. At the 13 seconds into the video, when she turns the phone's microphone towards the back of the car, you'll hear it get louder till she moves the phone back to the microphone pointing towards the windshield. It stinks because the sound is WAY louder in person when you are in the car than what this video is portraying. I'm going to try to take a better video tomorrow.
Video link:
Video - TinyPic - Free Image Hosting, Photo Sharing Video Hosting