Had the same rattle/tinny sound on my 2020 GT. It turned out to be the stainless shield with the single bolt holding it on.
That shield was never installed properly at the factory. There are rubber tab sleeves on the inner side of that shield. They have to be
engaged into the metal tabs welded to the muffler. Mine werent, and the vibration was bad. Took me 15 mins to get those rubber slots
lined up with the metal tabs on the inside of the shield as u cant see what your doing during the install. Once i got it right, the vibration
tinny sound disappeared. I knew it was that shield because when i took it completely off, the vibration also stopped.
When it warms up, it does get quieter for some reason. Maybe metal expansion???