2009 GT 5-speed. Hurst shifter, FRPP handling kit (shocks/springs/struts and stabilizer bars), and Steeda adjustable UCA. When I shift gears there is a pronounced slapping sound from the rear/center, around where the UCA bolts onto the frame. I checked the pinion angle after installing the UCA and it was within spec. Because I had never done it before, though, I took it to a trusted shop to double-check and to make sure I had torqued everything to spec. The sound is literally like someone slapping a butt--a big fat butt like mine. I suspect that something is actually hitting the underbody. First, has anyone else experienced this (the sound, not the butt slap)? Second, if something is hitting the frame how can I tell what it is when the car is moving? Finally, did I trust the shop more than I should have? IOW, should I take it somewhere else? Thanks in advance for your help.