Well, something inside the muffler on my 318i vert broke and was rattling really obnoxiously, so today I went ahead and replaced it.
Bought the replacement off everlast (thanks dude!) - it's a passenger side muffler from a 2005+ Mustang GT. It actually fit in pretty nice with a bit of fabrication (4 hours, lol) on the hangers and pipes. It would have been much easier to fit a driver side muffler (I had to flip the passenger side one to get the inlet and outlet on the right sides), but the price was right!
I haven't driven it yet, but from idling and revving it in the driveway it's a fair bit louder than the stock-ish setup that was on there before... but it doesn't rattle
Anyway, here's a couple of pics with the new setup - I think it looks awesome on the 318 valence with the small round (single) cutout.