I've noticed the whistling sound while braking on many cars, and its not vehicle related. Its more related to the tires and the road surface you are slowing down on. But then, thats just what I have observed.
I thought I was the only one who thought the whistling was cool! I absolutely love it. It sounds like a very faint diesel engine winding down. It's so subtle you can only hear it with the window down at low speeds.