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 CLK320 Failed Exhaust Catalytic Converter May Cause Rattle From Beneath Vehicle
Mercedes-Benz CLK320 Problem
Average Mileage: 42,680 mi (25,860 mi - 59,500 mi)
Visitor, 2001 Mercedes-Benz CLK320, 25,860 mi
One of the two small catalytic converters under the middle of the car is rattling. Diagnosis is a plate has come loose. Still working on an affordable fix.
Visitor, 2000 Mercedes-Benz CLK320, 59,500 mi
Rattling noise underneath car, dealer service says catalytic converter has failed. The dealer and corporate refuses to honor the 80K warranty claiming the car is more than 8 Year old.