What was found
(CAN) PILOT REPORTED LOUD ENG NOISE ON DESCENT OR PCL REDUCTION. ACFT GND RUN, PERFORMANCE CHECK FOUND GOOD. BORESCOPE INSPECTION OF CT GOOD. PROPELLER BALANCE CHECKED GOOD. REPLACED ENG BLEED VLV WITH SERVICEABLE VLV, NO IMPROVEMENT. CHANGED STARTER GENERATORS, TO HELP ISOLATE NOISE, NO CHANGE. UNABLE TO ELIMINATE UNUSUAL ENG NOISE BY BLOCKING AIRFRAME BLEED AIR SYSTEMS. IN CONSULTATION WITH MFG, REPLACED ENG AS A PRECAUTIONARY MEASURE. MFG CONFIRMED THAT THE POWER SECTION HAD ABNORMAL INTERNAL VIBRATION & WERE CONTINUING TO INVESTIGATE.