What noise does the APU typically produce during normal operation?

The APU, or Auxiliary Power Unit, in a C-130J is designed to provide electrical power and pneumatic energy when the main engines are not running. During normal operation, the APU produces a consistent hum and whirring sound. This sound results from the operation of its turbine engine, which is designed for efficiency and smooth operation.

The hum indicates that the APU is running steadily, while the whirring noise comes from various components such as the starter motor and fuel pumps engaged during operation. This type of sound is expected and indicates that the APU is appropriately performing its functions of providing power and starting pneumatic systems.

Other sounds, such as loud bangs, silence, beeping, or rumbling, would not characterize typical APU operation and could indicate a malfunction or other issues that would need to be investigated.