I have a 2000 SL500 and it has only 66,000 miles. Tonight I noticed a rattling noise coming from under the car. It comes and goes but starting to scare me. It's definitely not the engine and I don't think its the trans. The car shifts smoothly and effortlessly. The car seems to make the sound after I let off the accelerator or when you hit a bump. You can actually feel it when sitting on the passenger side of the car when your left foot is on the floor or leaning on the trans bump. It isn't suspension related either. Has anyone heard of this or something similar?

It appears to be the passenger site cat up near the manifold. If I bang it you can hear something rattling around inside.