What is the loud noise of a helicopter when the rotors are not yet moving?
The sound you're hearing is the APU (Auxiliary Power Unit) running.

Turbine (jet) engines turn at a much higher RPM than reciprocating engines. This means that an electric starter motor would have to be bigger and heavier in order to produce enough torque to get the engine moving fast enough. The bigger the engine, the bigger the starter (and the batteries to drive it). At some point, it becomes impractical to have an electric starter on a big turbine engine.

That's where an APU comes in. It's essentially a small jet engine. Small enough, in fact, to be started electrically. Once it's running, it feeds high-pressure air to spin the main engine fast enough for it to start. It can also provide electricity, run the A/C, etc.