We have a winner!
JCE, you're the man! It was a loose parking brake cable allowing the parking brake shoes to rattle. My mechanic had put little wooden shims between alomst every piece of metal on the suspension and both rear wheels before he could narrow the search.
This was difficult to replicate without the car moving, but also had to hoist the car in order to inspect.
Nevertheless he located it and I want to sincerely thank all of you for your timely attempts at solving the riddle.
01 S55 AMG Silver
01 CLK55 AMG Silver