had the same problem on my 190e.....I'm sure you can hear the rattle even more if you accelerate. The problem is most likely the "honeycomb" inside the cat is loose. Since my tags are good until 6/13 I had my friend take out the cat and I will just get a new one once I get closer to my next smog check date. a way to test it is to life the car up and go bang on the cat with your fist, engine off. You will probably hear it rattle inside.

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1990 190E 3.0L