1987 5.0 Muffler problem need advice

Hello all,
I was able to steal my uncles 87 mustang for the month with the promise I'd work on it. Unfortunately the car sat outside without running since last june (the last time I had it). Since last June I have had a new problem emerge. The muffler on the passenger has a extremely loud rattling noise. When I went to look at the muffler as the car was running I could see it flexing up and down almost as if rocks were getting thrown around in there. Would anyone have any suggestions as to how I could stop this rattling/flexing possibly without replacing the muffler? Thanks for the help