What was found
PILOT REPORTED, DURING THROTTLE ADVANCE ON RUN-UP, A LOUD RUMBLING NOISE AND SEVERE VIBRATION STARTED ABRUPTLY AS TORQUE APPROACHED 40 PERCENT TORQUE. INSP OF ALL SYS BY MECH FOUND NO PROBLEM AREA. TWO SUBSEQUENT RUNS PRODUCED NO REPEAT OF PROBLEM THIRD RUN, SHIP START VIBRATION AGAIN, VERY SEVERE, LOUD NOISE. AFTER SHUT DOWN, TEAR DOWN OF DRIVETRAIN AND CONTROLS, DISCOVERED FAN AFT CONTROL TUBE FOR ANTI-TORQUE CONTROL HAD RUBBED IN 2 PLACES. RUB FND ON FWD PART OF ROD THAT PASSES THRU FAN GRBX, OUTPUT GEARSHFT ID, REAR CNTRL TUBE, MINOR RUB CENTER OF CNTRL TUBE. SUBMITTER FOUND NO CAUSE FOR THIS CONDITION. MD HELICOPTERS INFORMED. (X)