What was found
AT 10000 FT THE FIRST OFFICER REDUCED THRUST TO FLIGHT IDLE TO INITIATE DESCENT. AT ABOUT THE SAME TIME A PUFF OF SMOKE ENTERED THE COCKPIT. THE SMOKE WENT AWAY ALMOST IMMEDIATELY. ALL ENG & ECS INDICATIONS NORMAL, NO CAUTIONS. FLIGHT ATTENDANTS DID MENTION THAT THEY DID SMELL SOMETHING, BUT IT ALSO WENT AWAY IMMEDIATELY. WE WEREN'T SURE IF IT WAS BURNING OIL OR A STUCK VLV BUT THERE WAS NO SMOKE AFTER THE FIRST INCIDENT. NO FAULTS FOR ANY ASSOCIATED SYS NOTED IN MDC. INSPECTED ELEC BAY & AFT EQUIP BAY. NO ABNORMALITIES NOTED, INSPECTED COCKPIT & CABIN LIGHTING SYS. NO FAULTS NOTED. PERFORMED APU BLEED AIR OPERATIONS CHECKED WITH LT & RT PACKS. ABLE TO DUPLICATE DISCREPANCY. PERFORMED ENG BLEED AIR OPERATIONS CHECK WITH LT & RT PACKS. UNABLE TO DUPLICATE DISCREPANCY. COMPLETED PACK BURN PROCEDURE TO REMOVE ANY RESIDUAL CONTAMINATION FROM DUCTING. ALL CHECKED GOOD IAW AMM 21-20-00, 21-50-00 & 36-10-00.