Looks like you got it figured out now huh?!?

 

The HR y-pipe definitely has a rattle/buzz when the temps and pipes are cold. Here's a video of my sedan with HR y-pipe, 05 sedan middy, and slightly modded airbox. Listen to the rattle on a 50% throttle take-off.



As for the buzz, make sure to check the front passenger seat belt clip (part the belt goes into). Mine can rattle depending on temp, humidity, and position of the clip when no one is in the seat. The sound is interesting and sneaky in that it sounds like it's coming from below the car.