Actually the SHO, and maybe Ford in general, are notorious for getting heat shield rattles. Can you localize the sound, is it always present, only at idle, or only intermittent? Is it only with brisk acceleration... Our cars have a very large anterior-posterior torque roll under throttle.
I'd start at one end of the exhaust and work to the other checking heat shields, exhaust pipe connections and exhaust hangers... If everything is tight look for signs of contact of the exhaust itself with anything else.... Beyond that it may just become a search..