Audio Pop

When I start my PowerBook up or wake it from sleeping and have the headphones plugged in it gives a small popping sound before the audio starts playing, is that a normal thing that happens?

Edit: The pop happens with system sounds too, e.g. moving something to the trash and the sound associated with it has a pop right before it plays, pausing a dvd for any period of time and pressing play again has a pop before the audio starts again.

yes.. same thing happens when i start up my mp3 player.. i think its just the energy moving into the headphones or something