APU have explosive sounds and rumble with vibration

[Morelaz]

AOG AOG AOG

Hello All,

We have had a nightmare already in 4 days.

The main issue is as soon are operable APU, with APU BLEED valves on (~20 psi), no any deviation, but as soon L/H or R/H ON (Single or both) the pressure increase till 40 psi and more, and after that, we have explosive sounds and rumble with APU vibration from APU inside itself (compressor
airflow with vibration, and airflow in the pneumatic duct behind the surge control valve to the exhaust).
During APU BITE Procedures no faults.
During the Engine run-up with the Air supply unit (Ground source), the engine was started perfectly.
With Engine bleed ON, L/H and R/H packs work perfectly. It means that with L/H and R/H packs and the engine's pneumatic system is all good.

Really it is an untypical issue and the first time for us. Please feel free to jump into the conversation, if you have any other ideas.
Your support will be appreciated.

Actions were completed:

APU Check Valve Removal/Installation/Visual check TASK 36-14-02-000-801; TASK 36-14-02-400-801.
Bleed Air Valve Removal/ Installation/Swap Test TASK 49-52-11-000-801; TASK 49-52-11-400-801.
Inlet Guide Vane (IGV) Actuator Removal/Swap Installation/ Test TASK 49-52-12-000-801; TASK 49-52-12-400-801.
Inlet Guide Vane (IGV) Actuator Operational Test TASK 49-52-12-710-801.
Inlet Pressure Sensor Removal /Installation/ Swap Test TASK 49-52-31-000-801; TASK 49-52-31-400-801.
Total Pressure Sensor Removal/ Installation/Test/Swap TASK 49-52-32-000-801; TASK 49-52-32-400-801;
Total Pressure Sensor Inspection TASK 49-52-32-200-801.
Delta Pressure Sensor Removal/ Installation/ Swap/Test TASK 49-52-33-000-801; TASK 49-52-33-400-801.
Delta Pressure Sensor Inspection TASK 49-52-33-200-801.
Surge Control Valve Removal/ Installation/ Swap/Test TASK 49-52-41-000-801; TASK 49-52-41-400-801.
Electronic Control Unit Removal/ / Installation/ Swap/Test TASK 49-61-12-000-801; TASK 49-61-12-400-801
Inlet Temperature Sensor Valve Removal/ Installation/ Swap/Test TASK 49-61-31-000-801; TASK 49-61-31-400-801.
Engine Bleed Air Isolation Valve Removal/ Installation/ Swap/Test TASK 36-13-04-000-801; TASK 36-13-04-400-801.
Fuel Control Unit Removal/ Installation/ Swap/Test TASK 49-31-11-000-801; TASK 49-31-11-400-801.
Electrical continuity checks all wires from INLET TEMPERATURE SENSOR TO ELECTRONIC CONTROL UNIT.
Electrical continuity checks all wires from the INLET PRESSURE SENSOR TO the ELECTRONIC CONTROL UNIT.
Electrical continuity checks all wires from TOTAL PRESSURE SENSOR TO ELECTRONIC CONTROL UNIT.
Electrical continuity checks all wires from DELTA PRESSURE SENSOR TO ELECTRONIC CONTROL UNIT.
Electrical continuity checks all wires from IGV TO ELECTRONIC CONTROL UNIT.
Electrical continuity checks all wires from SCV TO ELECTRONIC CONTROL UNIT.
Electrical continuity checks all wires from BLEED VALVE TO ELECTRONIC CONTROL UNIT.
APU full BSI was performed without any findings or FODs inside.
APU inlet with borescope checked for FOD, no findings.
Exhaust checked with borescope checked for FOD, no findings.
During APU run with APU generator ON, we have done full load to the generator with all possible electricity systems, no any deviations per parameters.

Really it is an untypical issue and the first time for us.
Please feel free to jump into the conversation, if you have any other ideas.