Had the same problem, it is where the X-pipe connects, and ensure the axle back is welded // clamped very well. Due to the increased airflow and unrestricted flow from the now non-existing catalytic conv. the axle-back system may "feel" secure enough but under the pressure the rattling can occur from shaking underneath especially when driving on uneven pavement. Easy fix, just let the local exhaust shop know the pipes are shaking under acceleration due to the increased/unrestricted airflow and see if they can add another hangar and go over the welds. FYI "off-road" mid pipe does indicate especially with headers NO airflow restriction, and mufflers will cause a degree of pressure which do generate more of this under acceleration. Off Road X pipes along with LT headers are designed for a muffler-delete setup for off-road or "track" application. Going with a muffler delete setup may remedy some of the shaking or rattling. Pypes and Magnaflow offer these axle backs. Cheaper would be to go custom from a good exhaust shop. This conclusion wasn't given by an automotive shop, a few friends in engineering helped out. So most techs in the shop wont be able to diagnose this. But no a weld would not produce a rattle, as the weld itself doesnt produce vibration, its the air escaping or in this case not escaping. ALSO with the O2 extensions being sent with the BBK products, being the wrong length; the rear sensors should be the only ones affected, and they can be turned off by a custom tune. Even if the correct length was sent the CEL would remain on due to the deletion of the catalytic converters.

I have wrote BBK three letters and so far only got back: Hope it works out for you and : we will forward your information to the R&D dept.

So I have contacted a professional custom exhaust place here in K.C.
Tomorrow I take my car over and they are going to first install a resonator in the exhaust. If that does not work or lessen the resonating sound then they can custom form a whole new X pipe for me. I hope that eventually it gets corrected. Other then that my car is perfect.
I am in the process of adding red stripes on it and maybe have a silver pin stripe next to the red stripe as well.