What was found
LHR - ON DESCENT AT 40K TO 37K FEET FIRST OFFICER NOTICED A PUFF OF DUST OR SMOKE IN COCKPIT, AND SAW PUFF AGAIN AT 37K TO 34K FEET. SMOKE HAD NO ODOR. SMOKE CLEARED PRIOR TO LANDING. LANDED LHR WITHOUT INCIDENT. NO INBOUND FAULTS NOTED. RAN LEFT AND RIGHT HAND PACKS AND ACCOMPLISHED GROUND TEST. NO FAULTS NOTED AND NO SMOKE OR ODORS THROUGHOUT COCKPIT OR CABIN. SYSTEM GROUND CHECKED NORMAL OPERATION.