What was found
ON GROUND, DURING PREFLIGHT, APU AIR ON, BOTH PACKS ON. SMOKE FILLING THE CABIN. HEAVIER IN AFT CABIN. TURNED OFF THE A/C PACK, SMOKE DISSIPATED OVER TIME, TURNED OFF APU. FOUND DEICE FLUID AT IN THE LEFT OF APU. COMPLIED WITH PNEUMATIC SYSTEM DUCT BURN OUT AND CLEANING OF THE APU INLET. PNEUMATIC SYSTEM. OPS CHECKED OK, NO ODOR OR SMOKE . RETURNED TO SERVICE.