What was found
AT FLT LEVEL 390, A SUDDEN, EXTREMELY LOUD SQUEALING NOISE FILLED CABIN, NO SECONDARY INDICATIONS OF A PRESSURE ISSUE AT THE TIME. STARTED A RAPID DESCENT AS A PRECAUTION, NOISE DID NOT DISSIPATE UNTIL BELOW 6000 FT WHEN CABIN & AIR PRESSURE EQUALIZED. COMPLETED CONTROLLER TO PNEUMATIC RELAY CONNECTION LEAKAGE TEST & VACUUM & PNEUMATIC RELAY TO OUTFLOW VALVES PNEMATIC CONNECTION LEAK TEST (CHECK 1 AND 2). BOTH CHECKS SATISFACTORY. COMPLETED A CABIN LEAK TEST & SECONDARY DOOR SEAL TEST. SECONDARY DOOR SEAL FAILED. FOUND SEAL BLOWING OUT ON LOWER DOOR. ADJUSTED DOOR/SECONDARY SEAL IN REF TO THE MM 52-10-01. COMPLETED A CABIN LEAK CHECK & SECONDARY DOOR SEAL. BOTH PASSED IAW THE MM 21-31-00.