Light thump heard under accel/deceleration

[gray87GT]

Since I installed most everything in my sig I have about 30 miles on it, and I noticed a new noise. When I start to move, or am rolling and hit it or back off there is a light thump from the rear somewhere. It's not a U-joint sound, it's kind of like a pop. I am wondering if it is the subframes. I have FMS subframe connectors and when I installed them they were tight up against that low circular spot in the floor boards, could it be my floorboard popping up and down from flex??? Anybody have any ideas?? Thanks

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87 GT, BBK shorty's, MAC High-Flow H-pipe, ASP pullies, BBK 70mm T.B., Cobra intake, no smog pump, Windsor Jr irons, Comp cams 1.6 rollers, F303 cam, FMS HD water pump, Tri-Ax, FMS subframes, Battle boxes, SSM liftbars, FMS King Cobra clutch, mass-air conversion, 73mm C&L, 24lb injectors, BBK AFPR, 3/8" no EGR spacer,MAC pro-dumps, FMS 3.73's, and other piddly sh*t.