Might want to check your upper and lower control arm bushings. It will sound clangish when those bushings go out. Or your isolators may be non-existant. So the springs may be recoiling and hitting metal on metal. But to answer your questions on the rear speakers, there is screws that hold your speakers in place.
2015 Race Red Performance Package GT 5.0 (435hp) stock
Still in the making Updates coming soon!