Many years ago the high frequency vibrations were attributed to some of the rubber seals between wing fillet/centre section and fuselage. More recently, vibration (moderate frequency) and felt through the floor in the cockpit, occurring between FL180-230 and speeds 280kts-310kts, can be attributed to excessive wearing on the TPFE (Teflon) bearings in the aileron hinges and even the hinges on the elevators and rudder.
Good luck.
N.M.