What was found
APPROX 10 MIN AFTER POWER UP AND STARTING THE APU A VERY DETECTABLE AMOUNT OF SMOKE WAS FILLING THE CABIN. IMMEDIATELY SHUT A/C DOWN, NO OTHER NOTABLE PROBLEMS. ENVIRONMENTAL NOT ELECTRICAL SMOKE. COULD NOT DUPLICATE DISCREPANCY. HOWEVER WHEN MAX COOL SELECTED SMOKE WOULD APPEAR IN CABIN. THIS WAS DUE TO CABIN ACM SEIZED. COCKPIT AIR CONDITIONING SYSTEM OPERATIONALLY CHECK'S GOOD IAW: CE750 M/M 21-11-00. REMOVED & REPLACED CABIN ACM (FWD) CABIN AIR CONDITIONING OPERATIONALLY & LEAK CHECK'S GOOD IAW: CE750 M/M 21-51-00.