ok, new everything for the clutch nothing old, all parts are gm, slave and master, the line has the drill mod, the clutch is a spec 3 and the flywheel is a centerforce. the headers are pacesetter with the pacesetter ofy, the car has 346.8 miles on it, but

whenever i get the car going the y pipe slaps the floor like tatatatatatatatata? and the car shakes? if i creep foward it feels ok and if i take off at like 2grand and blend it it sounds/feels great? is this part of the breakin? is something wrong? car only has 62k on it so shes nice???any ideas is this normal?