Speakers makes noise, but only when running with batteries..

Exactly as the title say: There is a low-but-noticeable “white noise” from the speakers but only when the lap is running from batteries :open_mouth:
If the laptop is plugged, there are no problems with sound, but as soon as I unplug it, the speakers starts to make that white noise even if they are muted. This white sound last up to when I change something in the sound configuration (it is enough to change the volume control…), then I hear a soft “plop” and the noise stops for a few seconds, but then another plop and the noise starts again.
I have a toshiba satellite p200-17d laptop, with intel sound-card (kde tells me it is a HDA intel ALC268), and openSUSE 11.1 with kde 4.1.3.
Any idea?