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! 

Steve Colwell  Legacy (no practice stalls, Advance Avionics AOA and Chelton)