As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WHEN TURNING ON BOTH PACK WE HEAR LOW RUMBLE AND SMELL SOMETHING ODD. DUCT PRSR DROPS SIGNIFIC RESECURED FLIGHT DECK AIR DISTRIBUTION SYSTEM MUFFLER/DUCT SLEEVE LOCATED AT DISTRIBUTION BAY REF. AMM 21-22-05-400-801, OPS CHECK GOOD, NO FURTHER RUMBLE OR SMELL NOTED.