[LAU] loud noise out of external soundcard in the bandroom when AC-powered

Hello list, When playing with my band in the bandroom, I'm recording the whole session with ardour and playing the phasex resp. zynaddsubfx via a midi keyboard. To do this, the outputs of my external soundcard (m-audio fast track pro) are plugged into the PA. Because the sound is very much amplified, an incredible noise can be heard when no one's playing, approximately as loud as two people talking (yes, it's very annoying!) - however, yesterday I noticed that this only happens when my laptop is AC-powered! If it runs on batteries, it's completely silent and there's no noise in the background. Now, before I start fiddling around, is this because of a poorly shielded powersupply? Or is this problem related to the shielding of the laptop? How can I build an effective shielding? Greetings Matthias