As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
INTERMITTENT THUMPING NOISE IN COCKPIT WHEN EITHER LEFT OR RIGHT PACK TURNED ON OR OFF. RECIRC OFF DOESN'T HELP. LEFT A-C PACK FLOW CONTROL VALVE, UPPER SUPPORT ROD ASSY FOUND TO BE WORN OUT. SUSPECT CAUSE OF HUMMING SOUND WHEN PACK IS TURNED ON. REPLACED SUPPORT ROD. ALSO FOUND GROUND AIR SERVICE FLAPPER STUCK IN OPEN POSITION. REPAIRED VALVE. NO ABNORMAL NOISES NOTED. NOISE LEVEL WITHIN LIMITS.