As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PILOT REPORTED IN CRUISE FLIGHT THE ROTOR NOISE CHANGED IN PITCH AND VOLUME SIGNIFICANTLY. MECHANIC PERFORMED VISUAL INSPECTIONS OF THE MAIN ROTOR SYSTEM, MAIN TRANSMISSION, BOTHE ENGINES AND THE TAIL ROTOR SYSTEM IAW EC135 AMM. NO DEFECTS NOTED. PILOT PERFORMED A SUBSEQUENT OPERATIONAL CHECK FLIGHT WITH NO DEFECTS NOTED. AIRCRAFT RELEASED FOR RETURN TO SERVICE.