As the airplane decelerated thru 89 knots TAS, once or twice a sound was heard that can best be described as a door closing shut firmly. It sounded like a landing gear door being slapped against the fuselage/cowling by the slip stream.

 

Jay, you could be hearing flow detaching and reattaching at the inboard wing.  I had this happen on our RV6 when practicing stalls.  The sound the RV made was a loud FLUPP! 

 

Easy way to test: Put tufts across the top of the wing (inboard both sides) and watch for flow separation concurrent with the noise.   If you see it….

 

Fred Moreno