I had the same symptoms when the cat in my '89 failed. The core came loose, and was rattling around for a few months. Eventually, it wore down enough so that it could move into the outlet pipe and plug it up - the rattling would stop when that happened. If the engine was turned off, the core would often come loose by itself, probably because the exhaust pressure stopped. Then I could drive for awhile, until the outlet plugged up again, and the rattle stopped.

Temporary fix was to pull over, crawl in the dirt, and kick the cat 'til it rattled again. (Hmmm...that makes me sound like an animal abuser.)

Permanent fix was to take the cat off, break up the core with a wrecking bar, and dump it out. Had to install a new cat later in order to pass the emissions test.

-Ron