You actually don't ahve to register. In the top right area of the page it says in rather small letter "Download Now", by clicking that you get to download it without logging in or anything. Yes, the sound does have to do with speed. As I am coming to a stop, the intervals at which it occurs get longer and longer, until it stops completely when my car stops.
It also slowly started to get louder, at first it was so quiet that it didn't bother me at all, now it roars. If I really get on the gas the sound is very rapid, which also led me to believe that it was wheel-related or something of that nature.
But do try to listen to the clip, you should be able to download it by clicking in the top right. The clip really does this justice as it's very hard to explain.