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.
Regards to if you lose the APU on push back there is no way to start as you have lost bleed preessure to start the engines. However if you were on stand there is a procedure to do a battery start if you have pneumatics but not electrics. Not the preferred method but it can be done.
Hope that this helps.
Spanner
