If it's a loud "clunking" noise or something along those lines, it could be the transmission mount...4-gens are notorious for breaking the transmission mount(speically when you do a lot of burnouts/get on it everywhere), and when you get on it the transmission jumps up and bangs on the bottom of the car...hell mine blew out in my 95 T/A about a year and a half ago...upgraded to a stronger "racing" mount and havn't had any problems since...you might want to look into that