I have the same thing dude, im pretty damn sure it is a bad join somewhere on the muffler/catback.
It seems like the same problem, i start the car with no noise but its like after 5 mins or so when the metal has warmed up i get the knocking rumble coming from the left hand rear corner. I'm replacing my entire exhaust system tomorrow so i shall tell you what i find as i pull things off.
Just a tip tho, after say a half hour drive park in ur drive way and keep the car idling and get someone to go give the muffler a not so gentle wiggle with their foot while you sit inside the car, positive you will hear the same noise. This set my mind at ease when i was thinking it was suspension related!