What was found
PILOT NOTICED A VIBRATION & METAL ON METAL BANGING NOISE WHEN ACCELERATING FROM GROUND IDLE TO FLT IDLE. PERFORMED EMERGENCY SHUTDOWN & NOTICED TAIL ROTOR APPEARED TO BE TURNING INDEPENDENT OF MAIN ROTOR. INSPECTION REVEALED THE T/R DRIVE SHAFT HAD SHEARED AT AFT END OF PLENUM TUBE. UPON TEARDOWN, DISCOVERED SPOT WELDS ON THE AFT SUPPORT OF T/R DRIVE SHAFT HAD BROKEN, ALLOWING SHAFT TO CONTACT TUBE. DRIVE SHAFT TUBE REPLACED. AFTER REPAIRS & DURING BALANCING TAIL ROTOR, FOUND ELASTOMERIC BEARINGS IN T/R FORK ASSY CAUSED OUT OF LIMITS VIBRATION. T/R FORK ASSY REPLACED. TAIL ROTOR BALANCED & AIRCRAFT RETURNED TO SERVICE.