As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
UNKNOWN SOURCE OF SMOKE FILLED COCKPIT AND CABIN SHORTLY AFTER TAKEOFF FROM ISP. A/C RETURNED TO FIELD, ISP. MAINTENANCE INSPECTED AND FOUND APU MAKING POPPING NOISE WHEN BLEED IS SELECTED ON. INOP APU BLEED SYSTEM UNDER MEL, RAN A/C, OPERATED ENGINE BLEEDS. FOUND NO DEFECTS. SUBSEQUENT TEST FLIGHT OF A/C PROVED SATISFACTORY. A/C RETURNED TO SERVICE. (M)