Reasons for UNUSUAL SOUND FROM TURBO END in HHP loco

UNUSUAL SOUND FROM TURBO END
a)DEFECTIVE TURBO MAKING HUMMING SOUND 
b)DEFECTIVE PLANETARY GEAR TRAIN IN TURBO. 
c)RUBBING OF IMPELLER WITH MAIN CASING
d)AUXILIARY GENERATOR DRIVE UNIT BACKLASH NOT ADJUSTED PROPERLY.
e)IMPROPER SEATING OF EXHAUST VALVE, BROKEN EXHAUST VALVE, INJECTOR DRIBBLING.
f) DEFECTIVE INJECTOR