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. However, the gear was retracted.

Jay,

I've seen several little 320/ 360 series planes go by with an inboard gear door open.  The hydraulic sequence valve was close to being adjusted but air loads changed things slightly.  I would look at your sequence valve and twist out that little screw a 1/2 turn or 2.  I suspect that sound of your gear door slamming shut was your gear door slamming shut.  Easy fix; hope it works.

Larry Henney

N360LH 890 hours