BLKSHDW

I just recently purchased a 2005 Z51. It has just over 118k miles on it. The guy before me got a custom Magnaflow exhaust put on it (I do not know who did the installation). I am not sure if it is something with the exhaust but I am getting a very loud banging/rattling noise while driving in the rear end of the car. What is odd about this noise, however, is that is doesn't occur when I first start the car and begin driving it. The banging/rattling only begins after a few minutes of driving and then it just gets worse and worse. I took it to a local exhaust shop and they said they couldn't seem to locate the rattle (they tried to search for it when car was cold). Whenever I turn, downshift, and cruise at or below 3k rpm, the sound gets worse. Also, once the sound starts, it vibrates all the way through the shifter. This sound is loud and very very agitating. Any ideas?

Thanks for your help!