What is this sound from Airbus aircraft before take off?
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.