Mystery noise. on 2002 Ford Mustang GT

I have a vibration type noise in the rear of the car. It is only when the car is in gear, typically at low speeds. Like in the drive thru. The car runs fine but the noise is getting louder. We cant seem to find it. It kind of sounds like a heat shield or something like that rattling.

Asked by for the 2002 Ford Mustang GT
With the cars engine off Kick the tailpipe and listen as the inside of the Catalytic Converter could have come apart and when they do it will have a rattle type noise when going slow or Idling