When you are checking the exhaust heat shield, check the rear exhaust box for a loose baffle.
I had one in the shop last week with the similar rattle and it was a loose baffle in the rear box.
The owner had previously been to half a dozen garages without success.
As a temporary measure I simply "reshaped" the box until a new exhaust is sourced.
Also check the aluminium shield along the floor pan tunnel at the fuel tank.
Should be very easy to trace once on the ramp.