Had just the same with my GT. Bought it about 3 weeks ago and spent a few hours trying to locate and sort the various squeaks and rattles.
The rear seats do take a bit of a slam to close the lock. Siliconed the locks and hinges.
The lower seats were making an little noise, silicone sprayed the hinges, this worked.
The little ash trays in the rear were a tiny bit loose, rattled slightly, so yup, you guessed it, siliconed those.
The tail gate had one of the rubber stoppers missing. Made one out of electric rubber gromits and super glue. That stopped quite a bit of noise, oh, and of course siliconed both plastic stoppers and springs.
The lock needed a good greasing.
The tools and spare wheel needed tightening up and siliconing. The little polystyrene top to the tool kit needed keeping in place, so I took the Alfa Emergency kit I took from my 159 and used the velcro ties to attach it to the plastic grill at the rear of the boot. This keeps a weight on the spare wheel lid.
After doing all those the rattles and squeaks have dropped significantly. So now I can REALLY hear the front upper arms chirping away!
Still loving the GT experience though.