A rattle that can't be found

A gotta problem here. Every afternoon I travel to my community college and finally park in the parking lot, there comes a huge rattling from the rear of my car. I leave my car running and went out to check it and sure enough, it was coming from the rear of the car. I first thought it was the heat shields but after moving the exhaust piping down with a stick, it wasn't that. So then I took my foot and pushed down on the tailpipe to lower the muffler and nothing. So then I thought it was my cat. but it wasn't coming from there. So it has to be something in the rear. I check my rear wiper, I checked the hatch, I even checked to see if my jack was moving around in the spare tire, but nothing. But it still comes from the rear. The only conclusion is that it's coming from my muffler, from inside. Could this be? It only happens when I drive the car for a long time when it heats up. Going to work (5 min drive) or to the mall, it doesn't do it.

yeah, could have a loose baffle.

Baffle, is that something in the muffler that lowers sound waves or something?

Yes. It also keeps the dampaning material in place. Most of the time they are either spot welded or pressed in, so they can come loose very easly, expecially with a little rust.

thanks man, looks like time for a new muffler!

Yeah, took it to a muffler shop and the guy took his fist and hit the bottom of it really hard and you could hear all the loose crap that was in there. He said it sounded more than one baffle. Said it was like maybe 3 or 4 that are completely loose. Thanks again.

That will be $39.95, I take cash, credit, money orders, and dates with your mother.