Rattles in side pipes can be unnerving. I bought a new set of stainless mufflers. Had welding company weld on both ends. Rattle ,Rattle, Rattle. Had pipes cut apart three time with no cause for rattle to be found. Shook, banged on, vibrated, re-welded and sure enough rattle rattle rattle. Finally went away. Best of luck to you!!!!!!!!!!!!!!
Later I found a washer that was down in the middle of where my four exhaust header pipes connect to the collector. May have been that I still do not know. My EM does not rattle now?