What was found
PILOT NOTICED NOISE AND VIBRATION EMMINATING FROM THE MAIN ROTOR SYSTEM BEGINNING 4 MINUTES INTO FLIGHT. NOISE AND VIBRATION INCREASED WITH AIRSPEED. PILOT ABORTED FLIGHT AND MADE A PRECAUTIONARY LANDING. MECHANIC WAS DISPATCHED AND FOUND DEBONDING BLADE TAPE (PU FILM) ON THE CENTER SECTION OF 1 EA M/R BLADE. OLD TAPE WAS COMPLETELY REMOVED AND AREA WAS CLEANED AND INSPECTED WITH NO DEFECTS NOTED. NEW BLADE TAPE WAS APPLIED, AND THE ACFT WAS RETURNED TO SERVICE.