Your cars chassis has just been replaced, I think? These rivets should have been replaced at the same time as they are part of the fixing of the body to chassis. When the cars are made there are usually a few unpainted rivets that were fitted after body paint in this area.
I also run the G56 exhausts and find that the double flex sections seem to reduce the amount of vibration transmitted to the silencers over standard, although it is hardly vibration free and they are heavier.
