What was found
CREW REPORTS LOUD NOISE FROM AFT CABIN DURING FLT CONTROL CHECK. LOUD NOISE IN AFT CABIN COMES FROM DEMAND ON APU BLEED AIR SYSTEM WHEN STARTING ENGINES. REPLACED APU MASTER CHIP DETECTOR HAD SILVER CHUNKS. ROUTED SCREEN FOR CLEANING. NO BYPASS EXPERIENCED. ELECTRIC STARTER SCREEN WAS TORN IN TWO QUADRANTS BOROSCOPE PLUG NOT BROKEN. IT WAS MISSING AND RED DIRT/SAND PACKED IN HOLE. APU REMOVAL WAS NOT REQUIRED.