I had some rattle when I installed my axlebacks on my GT last year. I went back through and tightened up all my hangar bolts and it seemed to go away. Something is probably just loose or they aren't quite adjusted right.
2008 Mustang GT Deluxe, Black, Manual Transmission, Black Cloth Interior
Show: Matte Black 10" Stripes, NXT Step Performance Axlebacks, Pypes M80 Mufflers (mid mounted), CDC Sequential Tail Lights, 14" Shorty Antenna, Ford OEM Plenum Cover, Ford Semigloss Black Painted Valve Covers, Leather Wrapped OEM Steering Wheel
Go: BAMA Tune, J&M Lower Control Arms