What was found
FLT NR 2134 - EMERGENCY LANDING, AT CRUISE POWER, A BANG WAS HEARD AND SEVERE VIBRATION FELT. THE FLT CREW PULLED POWER BACK TO IDLE AND VIBRATION STOPPED. ON ATTEMPT TO RESTORE POWER TO CLIMB, WHILE TRANSITIONING THRU 40-45% N1, VIBRATION AND BANG RESUMED. AIRCRAFT DIVERTED TO BDL. THE FLT LANDED NORMALLY AND TAXIED TO THE GATE. MX R&R NR 1 ENGINE. A STAGE 3 VSV SHROUD RETENTION PIN WAS FOUND LIBERATED WITH RESULTING DISTRESS TO STAGE 4 (HPT) HIGH PRESSURE COMPRESSOR BLADES AND OTHER DOWNSTREAM HARDWARE. WEAR OF THE S3 VSV SHROUD SEAL ANTI-ROTATION FEATURES LET THE SEAL STAGE ROTATE AND EXPOSE THE AFT FACING 9:00 RETENTION PIN ALLOWING ITS LIBERATION. REPLACED RETENTION PIN.