I have that problem, only at low RPMs when going WOT. But really it only seems to be present on Cobb, and I had never heard such noise with GIAC. However Cobb does have more torque at lower RPMs. Not sure what it is, the car drives fine otherwise and I have no lash/play in the diff or halfshafts. I read somewhere that the diff bushings may be soft. Could also be the exhaust.
PS: Why would you not speak the name of that forum member ?