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Hi,
If you've been underneath the Car and there are no loose screws or fasteners. And, the rattling noise is coming from a side muffler (Catalytic Converters), then the most likely cause is that your CAT has collapsed.
Inside the Catalytic Converter is a honeycomb-like matrix of individual cells. These are ceramic whose purpose is to chemically react with the Exhaust Gasses and alter them to less polluting by-products. Heat is an essential part of the reaction and these will run at over 900 degrees F.
Because of repeated heat cycles, as well as stress from bumps, and possible improper A/F Mixture, or just plain poor manufacture, this matrix can collapse.
When this happens, bits of the Matrix material can rattle around inside the CAT. Many will work their way through the exhaust to the Muffler, but many will not. The solution is a New catalytic Converter. A collapsed matrix can be so severe to restrict the Exhaust Flow, negatively impacting performance. If this is the cause, the only fix is to replace it.
But, I'm surprised that this condition wouldn't trigger the Check Engine Light as the OČ Sensor Readings must be out of whack as well. Are you sure it's the muffler and not the Heatshield surrounding it? These use a sheet metal screw and the hole in the Heatshield can expand to the point where this sheet metal screw can no longer fasten it securely and it rattles. This type of problem usually goes away once everything heats and expands...
Happy Motoring!... Jim'99
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