What was found
WITH THE APU RUNNING & BLEED AIR TURNED ON, SMOKE WITH A BURNING ODOR IMMEDIATELY FILLED THE COCKPIT & CABIN. RAN LT & RT ENGS TO ISOLATE DISCREPANCY, NOTED NO ODOR OR SMOKE. RAN APU & TURNED BLEED AIR ON, NOTED SLIGHT ODOR BUT NO SMOKE. REMOVED APU ACCESS PANELS & PERFORMED BORESCOPE INSPECTION ON INLET, NO OIL NOTED. REMOVED COALESCER SOCK & CLEANED WITH DETERGENT. PERFORMED ECS DUCT BURN OUT PROCEDURE, NOTED NO ODOR OR SMOKE. RE-INSPECTED COALESCER SOCK & NOTED SOCK WAS CLEAN & DRY, IAW FORM TC-680-21-120905M. INSTALLED ACCESS PANELS. OPERATIONS CHECK & LEAK CHECKED GOOD.