What was found
CLIMBING THRU 3000 FT CABIN FILLED WITH SMOKE EXITING FROM VENTILATION DUCTS. PILOTS CLOSED BLEED AIR VALVES & SMOKE SUBSIDED. ACFT RETURNED, NO INDICATIONS OF A PROBLEM OCCURRED. INSPECTION COULD NOT DETERMINE SOURCE FOR SMOKE. NO RESIDUAL SMOKE OR SIGN OF SMOKE REMAINED IN CABIN. INSPECTION OF APU, ENGINES, OIL & AIRFRAME SYSTEMS REVEALED NO REASON FOR SMOKE. EXTENSIVE INVESTIGATION REVEALED NOTHING. SUSPECT COOLING TURBINE MAY HAVE HAD AN INTERNAL LEAK, ALLOWING OIL TO ENTER VENTILATION SYSTEM & BEING HEATED BY BLEED AIR WAS CAUSE OF SMOKE. AFTER HI POWER GROUND RUNS & TEST FLT, NO FURTHER DIFFICULTY EXPERIENCED. ACFT FERRIED TO OXC, REPLACEMENT OF COOLING TURBINE ACCOMPLISHED.