What was found
DURING PREFLIGHT, HEARD AN EXPLOSIVE BANG FOLLOWED BY A NOISY APU SHUTDOWN AND IMMEDIATE INFLUX OF THICK SMOKE IN THE ACFT'S INTERIOR. REAR BAY HAD NO SMOKE PRESENT, BUT A LINEMAN OBSERVED SPARKS/FLAMES COMING OUT OF THE APU EXHAUST WHEN EXPLOSION OCCURRED. REMOVED AND REPLACED APU IAW MM 49-10-00 LEAK & OPS CHECKED GOOD.