What was found
WHILE CLIMBING OUT A HIGH FREQUENCY SOUND OCCURRED AT ABOUT 10,000FT. AT ABOUT 25,000FT A STRONG VIBRATION DEVELOPED AND THE SOUND INTENSIFIED AND CABIN AND COCKPIT BEGAN TO FILL WITH SMOKE. ALL ENGINE INSTRUMENTS LOOKED NORMAL ALTHOUGH CLIMB PERFORMANCE SEEMED SLOW. MADE DESCENT AND WITH POWER AT IDLE SMOKE DECREASED. WHEN WE ADDED POWER FOR THE APPROACH SMOKE REOCCURRED. ON THE GROUND DURING A MAINTENANCE TEST RUN UP THE LEFT ENGINE STALLED SEVERAL TIMES IN QUICK SUCCESSION. IMMEDIATELY REDUCED POWER AND SHUT THE ENGINE DOWN.