What was found
DURING WALK AROUND, HEARD LOUD "BACKFIRE" TYPE NOISE FROM APU EXHAUST AREA, COUPLED WITH A DRAMATIC REDUCTION IN AIRFLOW NOTED. HEARD X4 EVENTS AT 30 SECONDS APART, AND X2 MORE WHEN INTERNAL TO THE FLIGHT DECK. APU ON ELEC & BLEED AIR, EXT POWER AND AIR WERE BOTH DISCONNECTED. GROUNDED - YES. PER FIM 49-60 TASK 817 APU CURRENT FAULTS STILL HAS 49-61262 SPEED SENSOR 2 SHOWS OPEN CIRCUIT. SPEED SENSOR RESISTANCE TEST FAILED. SPEED SENSOR REMOVED AND REPLACED IAW AMM 49-61-21. APU BITE SHOWS NO CURRENT FAULTS. APU RUN CHECK NO ISSUES. NO LEAKS. INSTALLATION TEST GOOD.