What is this sound from Airbus aircraft before take off? When flying in an Airbus, I often (if not always) hear a weird sound before the aircraft starts to accelerate for taking off. From where exactly does this sound come from? Why doesn't it appear on other aircraft?
According to Ask the Pilot:


  Almost every frequent flyer has encountered this sound at one time or
  another. [...]
  
  It happens on twin-engine Airbus models: the A320 series (includes the
  subvariants A319 and A321) and the larger A330. [...]
  
  What you hear is a device called the power transfer unit, or PTU,
  which is designed to ensure adequate hydraulic pressures during
  single-engine operations. [...] Since it is activated only when the
  pressure falls below a certain level, the PTU cycles on and off, on
  and off, on and off.
  
  [...]
  
  Some Boeing aircraft also employ a PTU, but the operation is slightly
  different and it doesn’t bark like a dog.
  
  Another noise peculiar to Airbus models is a shrill, prolonged whine
  heard at the gate prior to departure and again after landing. This is
  an electric hydraulic pump used to open and close the cargo doors.