One thing to try is applying rudder each way during straight and level flight, and see if that causes the noise. If so, the noise might be caused by the lift pins working in their sockets.

There is also remote chance chance that it might be a loose joint between the steel mid-fuselage cage and where it attaches to the composite fuselage shell. It's unlikely unless the ship has a damage history involving a very hard landing or a ground loop. But you might have your mechanic or tech take a good look at it during the next condition inspection.

--Bob K.