What was found
(CAN) AT 21000 FT LOUD HISSING NOISE IN COCKPIT BUT CREW DID NOT KNOW WHERE, CABIN FAULT CAME ON , CABIN ALTITUDE WAS INCREASING, DIFF WAS MAXED OUT, CREW CARRIED OUT AN EMERGENCY DESCENT. 1) CDS FAULT CODES FOUND - CP CONTROLLER FAIL - CP/OF VALVE DATA FAIL 2) PRESSURE RUN CARRIED OUT IN MANUAL TASK 21-00-00-863-801 3) VALVE REPLACED IAW TASK 53-00-06-400-801. FUNCTION CHECK CARRIED OUT IAW AMM TASK 21-00-00-863-801, NO FAULT FOUND. UNIT HAD 145 HRS ON IT SINCE REPAIR, FROM A PREVIOUS FAULT AND NO FAULT WAS FOUND AT REPAIR VENDOR.