GlobuleSTP

I'm getting a faint buzzing sound in my rear channels in my 4.1s. I just go the speakers hooked up and have been listening to some MP3's with the volume knob pretty low (1/4th or so) The sound goes almost completely away if I have my surround knob cranked, it gets louder as I turn the surround knob down. Any ideas? (running an SB Live! Platinum w/ LiveWare 3.0)