What was found
APPROX 50 MIN INTO FLIGHT, SLIGHT VIBRATION FELT IN AIRFRAME. VIBRATION FELT EVERY 5 TO 6 MINS. LASTING APPROX.15 SECONDS. NO EICAS OR STATUS MESSAGES ASSCOCIATED. NO HIGH VIB INDICATION ON ENGINE. THE OUTFLOW VALVE APPEARS TO BE MOVING AND SEEMS TO COINCIDE WITH THE VIBRATION. VIBRATION WAS THE SAME @ FL400, FL380 AND FL360. DURING ILS APP. NO AUTOLAND DISPLAYED ON ASA. FOUND FWD CARGO BAY HEAT DISTRIBUTION DUCT DISCONNECTED, RECONNECTED DUCT, REPLACED ALL SCRIVETS AND CLAMPS. PERFORMED FLOW TEST AS PER AMM 21-00-00, PERFORMED FUNCTIONAL CHECK OF OUTFLOW VALVE, ALL CHECKS GOOD AS PER AMM 21-30-00.LP 1232590, ITEM M8: REMOVED L/H ACM IAW AMM 21-51-03. INSTALLED L/H ACM IAW AMM 21-51-03, OPS CHECK GOOD. S.N OFF: 15-3128, P/N OFF: 680640-4.ITEM M9: L/H PLENUM REMOVED AND NEW L/H PLENUM INSTALLED REF AMM 21-51-02.LP 1232591, ITEM M10: SECURED LOOSE DUCTS, INSTALLED NEW CLAMPS AND SCRIVETS AS PER AMM 21-40-00.