Bringing this thread back from the dead because I noticed a rattle similar to the one you describe when going over rough surfaces. Faint, metallic, from the front left, can be heard only with the windows down.
I did the old 'kick the tyre test', and sure enough, the front right wheel is silent, the front left wheel though produces a slight metallic 'clink'.
On my old Megane II I had the same symptoms and the problem was indeed the brake pad springs that were loose. I just don't remember it making such a noticeable rattle, of course the brakes on that car were half the size of the 200.
Are these springs available as a separate part or do I have to get a full brake pad set to get them?