As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
FLIGHT CREW REPORTED THAT AFTER BOTH ENGINES WERE STARTED IN MSP THERE WAS A VISIBLE HAZE THAT FILLED THE CABIN AND THE COCKPIT. REMOVED AND REPLACED BOTH RECIRCULATION AIR FILTERS PER AMM 21-25-02. RAN ENGINES. COULD NOT DUPLICATE AFTER FILTERS WERE CHANGED. INSPECTED DISTRIBUTION DUCTS. NO DISCREPANCIES NOTED. AIRCRAFT OK FOR SERVICE.