Did you figure out the steering rattle ? I think I have the same annoying problem. I bought the car at 40,000 miles and it had this problem then. I have driven it to 120,000 miles, but no one has seemed to know what it is, and I have asked at various repair shops over the years. I have had the bushing replaced and MB has attempted to correct, but it has never been fixed, and it really hasn't gotten any worse. The last time, they mentioned some major components inside the steering column that would bring the repair over $1,000. I'm not sure if this is correct.
As you described, it sounds like its close to the steering wheel, and although the steering column does not move around, the steering wheel will feel sloppy when moving it back and forth.