Sounds like a exhaust leak to me, put the 6 up and lay under it while its running and spray winded or soapy water on the pipes from the cats back to the tips if the solution bubbles there's the leak, if there's no bubbles anywhere it could ne in your headers, where they connect to th pipes. Hope this was helpful!
Sent from my MB520
2006 3.5L GT Coupe
Mods: Fully Custom Catback w/ Stainless Steel Piping & Flowmaster 80 series 1 in 2 out , CRAB Intake, Blue Calipers, Blue Spark Plug Wires, EE Headlights, White LED Dome, Map, and License Plate Lights, Black Badge Overlays