There is a piece of metal above the cat that acts as a heat shield. It's common for them to break a spot weld or two and flap....making a sound like you have check it out by grabbing the heat shield and see if it's loose. You probably should let it cool first :-).
4" Teraflex long arm lift. D30 tubed with chromoly axles, high pinion, powertrax no-slip and Warn Lockout Hubs. D35 w/ultimate 35 upgrade OX locker, 4.56 gears. NP231HD, Air on Board, Warn M8000 Winch.