What was found
HEAVY SMOKE ENTERED AFT CABIN TO THE COCKPIT WHEN THE APU BLEED WAS TURNED ON. THE CABIN GOT HOT AND A RUMBLING NOISE WAS HEARD. THE ACFT WAS SHUTDOWN. PERFORMED APU RUNS. PERFORMED ECU OPS CHECKS WITH NO DEFEFCTS NOTED. RAN APU BLEED AIR TO FULL HOT AND FULL COLD IN AUTO AND MANUAL MODES WITH NO SMOKE IN COCKPIT OR CABIN AT ANY TIME DURING 1 HOUR APU RUNS. REINSPECTED APU OIL LEVEL AND FOND SAME AS WHEN STARTED RUNS. ALL WORK PERFORMED IAW AMM 21-10-00, 21-51-00 AND 49-00-00.