Not really sure what your getting at here G-STAW.
Of course when the aircraft is starting engines packs are switched off (either manually Boeing or Automatically Airbus) At that point the aircraft EEC commands the APU controller to up the bleed pressure for engine start.
Consequently, there is more fuel going into APU and as such there will be an increase in volume which MAY be the sound which you hear.
ahh this helps alot, thanks