Curt,
I took my e320 to my mechanic, as I stated in an earlier post, and here is what I learned. At least on my "e-class", the parts I thought were the catalytic converters are actually resonators. The cats are farther up the line toward the headers. While this does not mean the cat(s) aren't bad, it does mean the noise is in the resonators. It could be that there is cat converter debris in the resonator(s), OR, it could mean a loose baffle in the resonator itself. I'm hoping for the latter, of course. So, the plan is to cut open the resonator (with a plasma torch) and see what's what. He indicated that the right resonator was the suspect. If cat converter honeycomb falls out, I guess I'll know I've got a cat going bad. If a baffle in the resonator is loose, it could be tack welded or removed. He said he could weld a sleeve (by the way, the sleeve is a BENZ part) over the baffle where the initial section was removed. The best part (if all this works as planned) is we're talking about 150 bucks, parts and labor. Either way, the rattle should be solved (temporarily if it's cat material that falls out after the surgery...time to sell, or permanently if it's a baffle or sheet metal). Our circumstances really sound similar, and while I don't know anything about a "c-class", I hope this info helps you, and hope you have a mechanic or a tech who is willing to do this work. I guess the cut could be made in the resonator with a cutting wheel on a 4" grinder instead of a torch.
The last thing I can tell you is, my mechanic is probably the best I've ever seen with european cars. I really trust this guy. Maybe the last thing I should say is, he kept the car, so I won't have a final verdict until tomorrow. I can keep you posted if you want. I'm hoping for the best....for both of us.

Dave