What was found
PILOT CONTACTED ME AFTER PERFORMING HIS DAILY PREFLIGHT AIRWORTHINESS CHECK. HE STATED THE TAIL ROTOR WAS MAKING A POPPING SOUND WHEN ROTATING IT BY HAND. I PROCEEDED TO REMOVE THE VERTICAL STABILIZER REAR FAIRING . I FOUND THAT WHEN HE ROTATED THE TAIL ROTOR BY HAND THE VERTICAL STABILIZER SHORT DRIVE SHAFT, HAD WORKING RIVETS AT THE LOWER FLANGE. ALL RIVETS ATTACHING DRIVE SHAFT FLANGE TO DRIVE SHAFT TUBE AT THE LOWER COUPLING WOULD SHIFT WHEN ROTATING THE TAIL ROTOR BACK AND FORTH BY HAND. I REMOVED AIRCRAFT FROM SERVICE. NO FLIGHTS WERE MADE AFTER PILOT REPORTED ISSUE.