What was found
LOUD FLT DECK WHISTLE STARTED ABOUT 3HRSINTO FLT. SOUND NOTPRSNT IN CABIN. INBD CREW STATED NOISE COMING FROM AIR VENTS, 15 MINUTES TOTAL, 3 HOURS INTO FLIGHT AND WENT AWAY 15 MINUTES PRIOR TO DESCENT. RAN PACKS FOR 40 OR MORE MINUTES, RAN 1 PACK AT A TIME AND ADJUSTED TEMPS WITH NO INDICATION OF ANY ABNORMAL NOISE IN FLIGHT DECK REF AMM 21-00-00. INSPECTED E AND E COMPARTMENTS AND AVIONICS FANS WITH ALL OPERATING NORMAL AND INSPECTED FLIGHT DECK WINDOW SILLS AND ALL SEALS INTACT. COULD NOT DUPLICATE NOISE, OTBD CREW GOING TO MONITOR. OK FOR SERVICE.