What was found
DURING "BEFORE START" CHECKLIST AND WHILE STARTING THE APU, SMOKE WAS NOTICED IN THE CABIN. THE APU HAD COMPLETED IT'S START SEQUENCE AND THE APU GENERATOR WAS "ON". THE "BLEED AIR" SWITCH WAS NOT ON. SHUT OF THE APU AND SHUT OFF THE THE REST OF THE EQUIPMENT IN THE COCKPIT. LOOKED FOR OTHER INDICATIONS OF A CONTINUING ISSUE BOTH IN THE CABIN AND IN THE ACCESSORY COMPARTMENT. NONE WERE NOTICED. CONSIDERED CALLING ARFF BUT DID NOT. R & R THE APU IAW MM 49-10-00. OPS AND LEAK CHECKED GOOD.