It is most likely a heat shield they put them above the catalytic converter and the muffler to deflect heat away from the body.With
the car on the lift you will be able to find it some times they crack
or a screw comes out nd then they rattle
most likely there is a rubber mount that has broke or something has become bent. It sounds like an easy fix to me. I would have to look at it personally. but to me it sounds like just part of the exhaust is rattling on the underbody of the car.