John, if it's the flywheel (MT only) that shows by going over about 3500-4000 rpms then keeping in gear when rpms get to about 2500 it feels like mild rumble strips. That's "normal" not right but "normal" I would ignore it. It's up to you about the rattle. I do suggest that before your dealer works on the car tell them about the exhaust and if they are friendly about that. Good luck and welcome there is lots of good info here.
also w/o insulting is it mild knock at low rpms?
2012 V6 traded in for a 2017 VW GTI.