panda, our 1998 E320 had similar symptoms. The rattling noise is caused by loose elements in the catalytic converters. I replaced the right assembly first, then the left side about 18 months later. If you have ramps, or a lift, it's a straightforward DIY job as the catalytic converter assemblies simply unbolt. The left side is more difficult due to the steering mechanism. I bought replacement assemblies from our local MB dealer, and gave them the old assemblies in lieu of paying a core charge. Expect to pay around $350 for each catalytic converter assembly.

If you tap on the catalytic converter assemblies with a rubber mallet when the engine is cold you should be able to determine whether one, or both, have loose elements. Or, you might try a stethoscope.

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Fred Hoelzle