My T-Bird also made a slightly irritating rattling noise. Turns out the substrate
had come loose in the pre-cat, and the exhaust pulses had bounced it around
so much that little pieces of it had broken off and made their way to the main
cat, while the disintegrating pre-cat slowly turned itself sideways inside the pipe
until it was almost completely blocking the exhaust flow from the left side of the
engine. Some new cats turned the chuff-chuff-chuff back to a vroooom.

A honeycomb catalytic converter that rattles might be crumbling. If it is it will clog the exhaust and greatly reduce power.