Get it up on the racks and check exactly where it is rattling. On one of my exhuast systems they didnt bend the pipe enough that goes over the rear lower arm of the suspension, so it hung down and rattled against the suspension arm.

I got a DC Sports header on mine too, never caused any problems for me. Probably something isnt aligned right with the piping, or it could be a bent heat shield. I had that problem too.