The rattle could be the shield rattling becuase it seperated at its welding points, or it could be because the ceramic element inside it has cracked. Catalytic converters have a fairly long waranty on them if I remember correctly. I had a problem which was similar to yours, the catalytic converter had broken and the sensors down the line had been ruined. This messed the engines settings up. They replaced the cat for free, but I had to pay to replace the sensors behind it.