As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
LOUD LOW FREQUENCY NOISE NOTED DURING FLT. LOUDEST ON LEFT SIDE OF AIRCRAFT COCKPIT. INSPECTED ALONG AIR DUCT RISER BEHIND FWD LAV SIDEWALL WITH RVI BOROSCOPE WHILE PACKS & RECIRC FANS RUNNING. NO DEFECTS NOTED. ACP SERVICE TIP 21-22-00 PARA 4 THRU 7 ALL CONDITIONS NORMAL. TESTED FLIGHT DECK NOISE LEVEL PER MM. FOUND DECIBEL LEVELS ACCEPTABLE.