Powerbook cracking sound

[persep]

Hi,
I have a Powerbook 1.5GHz 15.2" with mac os x 10.5.4 and yesterday while cleaning the computer I noticed something weird. With computer unplugged from the power source, i turned it on and increased the volume when the desktop come on. Before I could hear the alert sound when you change the volume there was another sound, like 3 lower volume pops, the normal sound it makes when you change the volume.
Then I turned off the computer plugged in all the cables and power source and turned on and then when I changed the volume I only hear the normal alert sound when changing the volume. It only happened when the computer was running on battery.
Has this happened to any other user?

Thanks

 

[persep]

Update:
I've tried the same thing on my other laptop, an IBook G4 1.33GHz with Tiger 10.4.11 and it does the same thing. 3 popping noises when changing the volume the first when not connected to the power source.
I wonder if it's some sort of warning sound because it's running on battery?