1975 Chevrolet Other muffler

A loud rumble is due to a leak. You're going to have to crawl underneath and look for that. There could be a hole in the catalytic converter, but those don't corrode very often. It's much more common to find a hole in the muffler, especially if there wasn't a tiny drain hole at the rear / bottom.

Some catalytic converters will rattle or make a thumping sound when the catalyst breaks loose, but in '75 GM used small pellets, not a solid substrate like everyone else used. Those pellets won't make noise.