When the engine is running, but the vehicle is not moving, a rattle heard from beneath the vehicle could indicate that one of the exhaust catalytic converters has failed. The substrate (the internal part of the catalyst that reduces emissions) can loosen and bounce around inside the canister section of the converter—the defective unit will need to be replaced. If this problem is ignored, the substrate can break down and reduce to a size that plugs the outlet pipe, which will cause a sudden loss in power.Google+
Car Problem Reports
Mercedes-Benz CLK430 Failed Exhaust Catalytic Converter May Cause Rattle From Beneath Vehicle
Mercedes-Benz CLK430 Problem
Average Mileage: 60,044 mi (11,000 mi - 91,000 mi)
Visitor, 2002 Mercedes-Benz CLK430, 85,000 mi
rattle under car, took it to a local muffler shop and said back passenager side cat is bad.
Visitor, 2002 Mercedes-Benz CLK430, 35,535 mi
I STILL THE PROBLEM. DOES ANYONE KNOW HOW MUCH THE REPAIR WOULD COST?
rattle on deceleration. replaced converters.
Visitor, 2002 Mercedes-Benz CLK430, 27,000 mi
Still have not fixed it. There is an EPA warranty on this, depending on the mileage, but they wouldn't cover it because my vehicle (which I purchased used) came in through Canada.
One of the rear cats is rattling and requires replacement.