As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
FLIGHT 3942 REPORTED SMOKE IN THE CABIN WITH THE APU RUNNING AND AN ASSOCIATED VIBRATION WAS FELT. APU WAS SHUTDOWN. MAINTENANCE REMOVED AND REPLACED THE AIR CYCLE MACHINE. SYSTEM OPERATIONAL CHECKS WERE COMPLIED WITH. ALL CHECKS GOOD.