The 747 Auxillary Power Unit, or APU, is a gas turbine engine that is housed in the tail section of the aircraft. The APU can supply electrical and pnuematic power.
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Type: Pratt and Whitney PW901A
Output: 1050KW (1408hp)
Weight: 404kg (890lbs) without oil and the 2 generators removed.
Speed: N1 24,625rpm / N2 31,200rpm
Electrical output: 2x 90KVA (2x152hp) 120/208volts @ 400Hz
Air output: 256kg/min at 57psi

The electrical output can power all of the 747 whilst it is on the ground if there is no external power available. All of the flight deck, the cabin, in flight entertainment system, ovens, chillers, you name it, it'll power it. Engineers can use the electric motor driven hydraulic pumps to test hydraulic systems without starting an engine.

Pnuematic air (fancy name for compressed air) from the APU's Load Compressor is used to start the engines via the engines' pnuematic starters (air driven motors). It can also be used to operate the air conditioning packs, keeping the cabin cool in the summer months or warming it in winter if there's no external air conditioning available at the airport gate. The 747 also has pnuematically driven hydraulic pumps, another way to run hydraulics without running an engine.

Typically, on the ground at the airport gate, electrical power is supplied by a ground power supply and preconditioned air is fed into the cabin by a ground air conditioning unit. Before leaving the gate, the APU is started then ground air and electrical power is removed. The APU powers the aircraft during pushback and the pneumatic air is used to start the engines. Once the engines are running the APU can be switched off.

The 747's APU is not designed to be used in flight, or at least not in this aircraft. This is considered unnecessary in a 4 engine aircraft.

The noise at the very beginning, before the start initiates, is the sound of the actuator opening the APU inlet door, the exhaust from the APU exits via the tail cone.