Rattling noise after header/cat install

well i finally got to installing the header and catco hiflow cat. but now when im driving i hear a buzzing or rattling noise. maybe its always there, but i dont hear it until about 2500rpm and up. whats weird is i dont really hear it if im in park or neutral and i rev the engine.

i tightened everything as well as i could, so i dont see how anything could be loose. but its really annoying. anyone know what it is??? or has anyone had a similar problem arise??

ok, i was able to pay closer attention to everything on the way to work. and i noticed that even at high speeds, i only hear the rattling when im accelerating. when i was crusing at 80, all i heard was muffler, as soon as i hit the gas the noise came back.

i can hear two new noises since the install. one i believe is due to the highflow cat. got a little louder and when i rev it sounds a little like running off headers. but still that damn rattle. ill try and get a sound clip of it for anyone that can help.

thanx