I've got the same problem. I installed the handling package from Steeda.. new shocks, struts, springs, sway bar, and strut brace, but the rattle in my rear end has gotten worse. It's been bothering me for a while. I pulled off the wheel.. checked the top shock mount. It's tight. I've checked inside the trunk.. nothing loose there. The rattle is progressively getting worse and it's driving me nuts. Don't get me wrong.. I love the car. But I'm probably gonna trade it for a new Stang in the next year or two. The car has to make it that far first. I've had NO OTHER PROBLEMS with this car. Since this pic, I've also installed the chrome fog light trim and put chrome pinstriping on the eyelids.