As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
VERY LOUD BUZZING NOISE FROM LEFT A/C PACK WHEN USING ENGINE BLEED AIR SOUND NOT PRESENT WHEN USING APU BLEED AIR. SOUND SEEMS TO COME FROM RIGHT SIDE GLARE SHIELD IN FRONT OF F/O. FOUND A LOW BUZZING NOISE WHEN RUNNING APU WITH RIGHT AND LEFT PACKS ON. FOUND NOISE TO BE COMING FROM NEAR F/O WINDOW SUNVISOR TRACK . FOUND LOOSE SCREW ON SUNVISOR TRACK. TIGHTENED SCREW PER AMM 20-11-00. NOISE APPEARS TO BE GONE.