What was found
INVESTIGATED FLIGHT CREW REPORT OF HUMMING/WHISTLING NOISES COMING FROM AFT CABIN CEILING AREA. FOUND SIGNIFICANT LEAKAGE FROM BETWEEN ADF ANTENNA ASSY AND FUSELAGE SKIN. UPON ADF ANTENNA REMOVAL NOTED AFT SPACER INSTALLED INCORRECTLY AND GASKET CUT TO FIT INCORRECT INSTALLATION. RE-ORIENTED SPACER IAW SERIES IPC 34-51-15-01 AND REINSTALLED ADF ANTENNA ASSY WITH NEW GASKET. LEAK CHECKS OK, NO ABNORMAL NOISES NOTED WITH CABIN PRESSURIZED. TRANSIENT ACFT, RECORDS NOT AVAILABLE, BUT SUSPECT ANTENNA INSTALLED INCORRECTLY SOME TIME PREVIOUSLY, AND TOOK REPEATED PRESSUIZATION CYCLES FOR SEALANT TO FAIL. RECOMMEND MFG INVESTIGATE FURTHER OR ISSUE A SB TO DETERMINE IF INCORRECT ADF ANTENNA INSTALLATION PROCEDURES ARE BEING PERFORMED AT MFG, OR IF THIS WAS A 1-TIME OCCURENCE.