What was found
DFW - FLT 2155 - SHORTLY AFTER LEVEL OFF AT FL370 GOT CABIN ALT WARNING. CHECKED CABIN PRESSURE AND IT WAS 10,000 FEET. DESCENT INITIATED AND CABIN IMMEDIATELY BEGAN A DESCENT AS WELL. ONLY THING UNUSUAL NOTED JUST PRIOR TO INCIDENT WAS A HOWLING/SQUEALING TYPE NOISE WAS HEARD IN THE COCKPIT FOR ABOUT 1 MINUTE. NO SUCH NOISE WAS HEARD BY FLIGHT ATTENDANTS IN THE MAIN CABIN. BITE CHECKED NR 1 AND NR 2 CABIN PERSSURE CONTROL, PSI'D AIRCRAFT, FOUND LEFT PACK SUPPLY DUCT CLAMP BROKEN. REMOVED AND REPLACED CLAMP, PSI'D AIRCRAFT. OPS CHECK AND LEAK CHECKS OK. OK FOR SERVICE. (M)