I had a bad CAT on a 1998 E320 that was fixed about 6 months ago under the 8/80000 federal emissions warranty after some discussion with the service supervisor who initially said it was not covered; but, we got it all sorted out.
It took me a few service visits to get it identified because it NEVER happened when I left the car. I made arrangements to drive with the service manager. This way I could ensure the problem was occuring.
Mine only did it when the car was VERY warm from driving on the expressway. Even idleing for a mere five minutes gave the CAT enough time to cool down and eliminate the problem. When it was WARM I could just lightly rev the motor and it would rattle every time the motor was idleing down.
Just find a protocal where you can consistently reproduce the problem and meet with service manager. I went as far as to video tape (with sound) how I reproduced the problem so at least the mechanic knew I wasn't imagining this it case it decided not to act up.