As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
HUMMING AND VIBRATION COMING FROM DOOR 3L. INSPECTED AREA, NO OVERHEAT, OR LEAKS, HUMMING VARIES WITH APU PNEUMATICS OFF OR WHEN PACKS ARE TURNED ON. SUSPECT AIR PRESSURE BUILDUP IN DUCTS WHEN NO DEMAND CALLED FOR. THIS IS A KNOW CONDITION. INSPECTED PAX AND ALL IS NORMAL.