What was found
(CAN) AT FL300, TOP OF DESCENT, AS THRUST REDUCTION COMMENCED, LARGE BANG WAS EXPERIENCED BY THE FLIGHT CREW WITH FLUCTUATIONS IN NR 2 ENGINE PARAMETERS. ENGINE WAS IDLED FOR THE REMAINDER OF THE FLIGHT AND SHUT DOWN UPON LANDING. MX PERSONNEL CARRIED OUT TROUBLESHOOTING AND THE PRESSURE RATIO BLEED CONTROLLER WAS REPLACED. THE PRBC HAS BEEN SENT TO AN OVERHAUL FACILITY AND AWAITING COMPLETE STRIP REPORT TO VERIFY DEFECTIVE PRBC.