If your S430 has secondary cats, as my E430 does, and if the secondary cat is broken (and thus causing the rattling) you can also gut it and be done.

The secondaries are downstream of the O2 sensor and do not significantly impact emissions, and definitely do not impact emissions testing. The only noticeable effect of gutting them is improved fuel economy, a bit more power, and a slightly more aggressive exhaust note.

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black