What was found
NON-ETOPS FLT 280 HNL-KOA: DURING TAKEOFF ROLL AND INITIAL CLIMB, CREW NOTED A LOW TONE REVERBERATING SOUND THROUGHOUT THE AIRCRAFT. THE SOUND PROCEEDED TO GET LOUDER AS THE AIRCRAFT CONTINUED TO CLIMB. CREW NOTIFIED ATC AND RETURNED TO HNL. MAINTENANCE REPLACED THE RT 35O CONTROLLER, 35O SENSOR, AND 35O CONTROL VALVE. MAINTENANCE PERFORMED ENGINE RUN WITH SOUND STILL PRESENT BETWEEN 1.40 AND 1.80 EPR SETTINGS. MAINTENANCE THEN REPLACED THE RT WATER SEPARATOR AND DIFFERENTIAL PRESSURE REGULATOR. MAINTENANCE PERFORMED ENGINE RUN AGAIN. NO DEFECTS WERE NOTED. AIRCRAFT WAS RETURNED TO SERVICE.