Probably a bad 'cat.....

Usually is. Sometimes you get lucky with a bad hangar or heat shield that can be peeled away though. Alot of catalytic converter rebuilders popping up due to exorbitant costs of replacement. Worth a google in your area.

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1985 500SL Euro w/ AMG bits 130k
1984 300SD Turbodiesel 192k
1980 240D Stick China 188k
2001 CLK55 AMG 101k
2007 S600 Biturbo 149k Overheated Project, IT'S ALIVE!!!

rattle could be a heat shield that's fallen away from the body, and is rattling against the exhaust. i've had that happen before.

will try to look underneath soon. A bit wet here today
Any cat rebuilders in DFW area? What is a fair price? I want to be ready if needed

lizem, as I am sure you are aware there is nothing to see when you look under your S430, but you should be able to detect the source of the rattle. If it's the catalytic converter on the driver's side, recommend buying a new assembly from a MB dealer and installing it yourself. Should be a simple bolt-up if it's like our 1998 E320. There is an Oxygen sensor before and after the catalytic converter.

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Fred Hoelzle

I was going to look underneath to make sure nothing obvious was loose.
Is it known that OEM cats are clearly better than aftermarket? I am happy to use them if so.
I have called around and muffler shops seem to charge $180-250 for weld in varieties

Good test for bad cat is hit it with a rubber mallet. You should be able to find the bad one. They are probably not all bad, just one.

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

Like mpolli said above, if you can get under the car and tap the rear cats you should be able to determine which one(potentially both) is loose.

I'm a big advocate in using factory solutions over cheap solutions so my vote is for factory cats. and for factory parts in ~99% of scenarios actually. unless there is an aftermarket solution that is far and away better than factory and doesn't cost 10x more than factory

I would be careful about using an aftermarket cat. Not always, but a lot of the time they can trigger an "efficiency" fault code. Then you may have to worry about passing emissions, depending on where you live.