Here's some info on the Cat ( please note about misfire and the Cat:
The catalytic converter, has the job of altering pollutants in the car's exhaust gas. The catalytic converters do this by chemically altering the harmful chemicals present in exhaust gases, such as oxides of nitrogen (NOx), unburned hydrocarbons (HC), carbon monoxide (CO) and oxides of sulfur (SOx) into harmless variants of these chemicals, such as carbon dioxide (CO2) and water (H2O). For a more in-depth description of the chemical reactions in exhaust gases, refer to the beginning of this section.
Two things can to destroy the catalyst: the use of leaded gas and excessive heat. The use of leaded fuel was the most common cause of catalyst destruction. The lead coats the thin layer of platinum and palladium (that actually promotes final combustion of the almost completely burned hydrocarbons that enter the catalytic converter). The coating keeps the mixture from actually contacting the noble metals. The lead may also cause the passages of the "substrate''-the material that carries the noble metal coating-to clog. This would cause the car to run at reduced power or even stop altogether.
Excessive heat results in the converter when raw fuel and air with a high oxygen content enter the catalytic converter, which greatly accelerates the combustion process. This most often occurs when a spark plug wire is disconnected. Excessive heat during misfiring due to poor vehicle maintenance and prolonged testing with the ignition system wires disconnected are two common ways a catalyst may be rendered ineffective. Test procedures should be accomplished as-quickly-as possible. The car should be shut off whenever misfiring is noted. Misfiring due to extremely lean or extremely rich mixtures will also damage the catalyst.
While the catalyst itself is a maintenance-free item, it should be understood that long life depends completely on proper fueling and good maintenance.
Sound to me like the "substrate'' that's in the Cat has melted and broken off and is rattling around in there.