My '87 Sammy has developed a rattle on hard acceleration.  It sounded like a loose heat shield somewhere, but on further investigation, it seems that the core of the catalytic converter is loose in its housing.  The entire exhaust system is tight as a drum, but when I thump on the cat, I can hear a rattle.  Do I need a new cat?

Sounds like you do need a new one. If those loose pieces plug it up, it may not run and won't run very good if if does.

change it quick.... I waited and drove mine for awhile. It ended up blowing pieces into the muffler. Then I ended up having to change both.