Have you looked here? http://www.need2speed.web.com/
Dean has a nice site with a lot of 500E/E500 related data, maybe we can convince him to start a registry??
I cannot help with the "cluncking" sound as I have not heard of this on a 500E before. Has the car ever been damaged in the rear before?
Benzmac may have a clue with some more vehicle history maybe??