"Slapping Sound" is a new one...
How loud is it, can you isolate front/rear or L/R?
Is it possible you moved the rear E-Brake cable and its slapping against the leaf?
Up front, did everything go together normal, hats are all sitting the way they should?
Is the rubber bump on shock shaft all good?
Not sure about É brake cable. Rubber bump stops on shafts are good.
will attempt to take a video to capture sound if possible and report back.