I recently purchased a used 2004 C240 Wagon for my wife. Car has ran like a champ for three weeks. Recently a rattling noise has started under the car, getting louder when stopped but goes away ( or cant hear it) when driving. The car has 17K miles on it and under warranty. Could it be the cat converters shot already? Any other possible causes? Car drives fine and fuel mileage seems to be non impaired. If cat converter is the problem, is this common on a 04? Thanks in advance.