I had a rattle that turned out to be from the second cat. on the left side. replaced the cat. and noise was gone.

The first time I heard the noise was when going over a bridge from start, under light torque. I had to search hard while on a rack with brake on and engine under torque "Power braking". It was hard to find even then.Tthis was on my SLK32 AMG with 60K miles.