What was found
SAME LOUD BUZZING SOUND HEARD FROM INSIDE THE COCKPIT AND VIBRATION FELT THROUGH FLOORBOARD IN OUR FEET. SOUND IS LOUDEST IN FLOOR NEXT TO LIFE RAFT AND LUGGAGE AREA. SOUND NEVER COMPLETELY GOES AWAY. VIBRATION IS PROMINENT IN OUR FEEL WHEN SOUND IS PRESENT. SOUND/VIBRATION BEGAN ON CLIMB OUT AT FL-120, WENT AWAY AT LEVEL OFF FL-310. CAME BACK ON DURING DESCENT AT FL-230. WENT AWAY AT FL-210. CAME BACK AT FL-170. WENT AWAY AT FL-130. NO CONFIGURATION CHANGES OR SPOILERS WERE USED WHEN SOUND/VIBRATION CAME ON OR WENT AWAY. ONLY PITCH CHANGE WAS AT LEVEL OFF AT FL 310 WHEN SOUND/VIBRATION WENT AWAY. AMM: 49-15-05-6, INSPECTED APU AIR INTAKE PORT ASSEMBLY, INTAKE DOOR AND INTAKE DOOR ACTUATOR PER AMM 49-15-05-6. CHECKED FOUND NORMAL.