I don't have a clue. I had the same thing happen on one of my Benzes, but I doubt that you are having this trouble with an S500. Bolts had come loose on my lower front control arm and when I'd brake and turn, the tire actually came back and scraped the front fender.
I'd suggest getting someone to walk beside the car as you brake around a cul-de-sac and see if between the two of you, you can locate the noise and its source. Let me know what you find.