My cat-back occasionally would bounce against the crossmember (being reinstalled in the attached pic, courtesy of:
http://www.gmhightechperformance.com/camaro/tech/1009gmhtp_2010_chevrolet_camaro_catalytic_converte rs_headers_intake/photo_19.html):
We just used stainless steel washers to space it out a bit (the kit in this pic came with spacers).
Just another thing that could be causing the sound (or likely will in the future after driving it a bit).
-LI-