Originally Posted by JAdamz
I was actually hunting this down for a couple weeks too a few months ago. Assuming you're having the same problem, turns out it was the metal black shield that covers the bottom of the mufflers hitting the bumper mounts at the back of the car when the exhaust warmed up and expanded. You can easily tell if that cover is hitting the mounts by looking for worn away areas toward the top of the shield where it curves upward toward the car. I resolved it by tracing out notches in the area where the mounts could potentially hit when the exhaust warmed up, and Dremel'ed them out.
Yup, this is it. I had this same exact problem which took me a while to figure out. You'd be surprised how much the exhaust expands when they heat up. I ended up taking off the cover on mine.