Be careful about replacing OEM shocks with after-market shocks if you have the self-leveling system. The Bilsteins (or any other brand not intended for self-level) are not going to work. If you do have self-leveling (and I suspect you do), the leaking shock definitely needs to be replaced. It could also be the source of the noise. I attached a link to my web page that describes an issue I had with my suspension...
http://v12uberalles.com/Rear_Struts.htm
I initially thought the noise might be coming from a bad bump stop on top of the strut. I pulled the strut out, saw that the bump stop was indeed worn and rattling, and replaced it. I put the strut back in, but the noise was still there. I grabbed the questionable strut, had my son push down on the car, and could feel something inside the strut creaking / snapping. Once I replaced the strut ($$$ for self-level and ADS), the noise went away.