OK i have borla and flowtech headers. Once I installed the headers, i got this horrible rattle. I can hear it over the radio. Its coming from the rear directly under the spare tire and jack area. I tightened the jack and spare tire so I know its not that. Its starting to kill me its so annoying

 

The I-pipe arch over the axle, buzzing the gas tank
heat shield? That's easy to let happen when you drop
the exhaust. Have to feel for it with the car on the
wheels, less than half an inch clearance and you may
hear it hit when the engine vibrates.

 

You sure the Y-Pipe is not banging against the K-Member. That seems to be a really popular problem with installing headers.

 

Mine did that too. Adjust the pipe that goes over the axle to provide for more clearance and that should solve the problem. Check the clearance on the gas tank heat shield and muffler .