I had this sound with the stock y-pipe, but it got worse with an aftermarket y-pipe (Warpspeed). Only hear the rattling sound inside the car and only around 1500 to 2500 rpm when driving. No type of sound from the outside. Maybe its something with the center part that holds the radio and stuff? I've heard that warpspeed pipes Supposedly hit or touch the cross member.

 

It's not the y-pipe. came to a conclusion that its something inside the car. Can't figure out what it is. I'll have to take a closer look tomm. after work.