Any suggestions for this audio problem?

[Techfan26]

Since I've had my powerbook g4, I've used a Y Audio Cable to hook it up to my stereo with no problems. I recently bought a monitor and I have it hooked up to my powerbook via DVI. The only problem is, I now have a deep humming noise coming from my speakers. I tried unplugging the dvi cable from the powerbook and the hum stops. So, I'm assuming the dvi cable is causing it somehow when plugged into the powerbook. Can anyone think of a reason for this? As soon as I pull the DVI cable out, the noise goes away. Thanks.

Also, I unplugged the monitor's power cord and the hum is still there. So, I'm thinking it has something to do with the DVI cable or mini dvi-dvi adapter.