As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AT FL 180 WHEN APU GENERATOR PUT ONLINE ON LEFT SIDE VISIBLE SMOKE AND BURNING SMELL IN COCKPIT. APU BLEED WAS CLOSED. SMELL RETURNED AT 7000FT ON DESCENT. REMOVED AND REPLACED HEP A FILTERS LT AND RT ACM'S INSPECTED BEHIND P4 AND P5 PNL'S AND UNDER APU LWR SHROUD. PERFORMED OPERATIONAL CHECKS AND LEAK'S. NO ODOR DETECTED.