Hey Scoop..I had a similar issue with my Magnaflow exhaust. Try taking off the bracket that clamps the two pipes together at the 110 degree bend (I think right after the secondary CATS) and see if that takes away that vibration/rattle you hear away.I have the M Performance exhaust and the main comment I have about it is it sounds amazing.
There is however a catch (and I wonder if others have noticed this as well). It has to heat up to stop rattling. I have taken it in to BMW service and they said it's normal. But for the first 5 minutes after startup it is hella loud.
If you can however stand that 5 minutes, the sound it makes when downshifting is life changing.