97 hardbody

You have no shrouds over the exhaust system in that area. What you are showing is a muffler, and it appears to have been replaced with an aftermarket muffler fairly recently, due to its good condition and the universal pipe clamp mounted directly behind the bolted flange. I don't see that thing that has a tan woven cover around it on my truck, but yours doesn't look too good. On my truck, the cat is in the exhaust system directly below the driver's seat and there was an 18", or so, long shroud over the pipe between the engine firewall and the cat. That is the shroud that rattled and I tore off.

It probably isn't the muffler then, unless the pipe connection came apart at that new pipe clamp. Put the truck in park (if auto) or put it in neutral and set the handbrake (if stick). Start the truck and get under and feel or listen for a leak. Feel first, before the exhaust gets really hot, then listen. If you have no help, kneel or lay next to the open driver's door and reach in to press the gas pedal with a stick or something long enough, so your head remains outside the truck. The noise should get louder and you may be able to determine where the noise is coming from. Otherwise, take it to a garage to have it inspected.