after i had my exhaust done, i developed a distinct metallic rattle that wasn't consistent at any particular rev range from day to day. when i got underneath the car i had a poke around, and discovered that where the resonator is, theres a metal strip that goes across the gap in the actual body of the car. as the exhaust pipe was fatter, it reduced the clearance between the pipe and this strip to about 2-3 mm. when the engine was under load it would move the exhaust pipe, causing it to occasionally hit and rattle on this strip. i just took it off, bent it slightly, filed larger holes in it for the mounting bolts and put it back on, and the clearance is just under an inch now. no issues since! maybe check that? the strip is painted black, so if it is the pipe vibrating against this strip, the paint will be scratched where it makes contact.