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 E320 Failed Exhaust Catalytic Converter May Cause Rattle From Beneath Vehicle
Mercedes-Benz E320 Problem
Average Mileage: 159,571 mi (66,000 mi - 210,000 mi)
Visitor, 1998 Mercedes-Benz E320, 130,000 mi
I PUT A SCREW IN IT
Visitor, 1999 Mercedes-Benz E320, 66,000 mi
rattle coming from the canister after the cat.
Familiar rattle due to a faulty catalytic converter. The engine power is unchanged and it still passes the California smog test. Currently, the rattle has stopped so until the mpg or engine power significantly changes for the worse I'll keep driving until the wheels fall off the chassis. I've looked into options to replace the catalytic converter but the cost still isn't reasonable enough to proactively fix the issue at this stage.
Visitor, 1998 Mercedes-Benz E320, 190,000 mi
Rattle in cat pipe and falling apart. Mercedes Benz replaced cat pipe and rattle was louder 2 days later.
Visitor, 1998 Mercedes-Benz E320, 141,000 mi
Rattle inside the exhaust pipe after the converter when engine is running. Not fixed.