I hit the exhaust system with a rubber mallet. It sounds like there is a long (maybe 12"?) piece of metal resting in the header pipe. I get it to clang when I hammer along a foot long length of the header pipe between the cat and the enlarged (looks like a small football) portion of the header pipe. How could something big get in there?

I looked carefully at the cat. It looks like it was a replacement because each end of the cat has two circumferential welds for step down adapters. Furthermore the Cat shows no MB marks and has the following stamped into the steel shell:
CA/CE 94007
07/03 36398.

Would the 07/03 be the date? If so that is pretty new for the cat to be breaking up.

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1972 450SL
1982 300D Turbo