Ground Loop Isolator (Score 1)

I had a similar problem in a home recording studio and fixed it using a Ground Loop Isolator. I have heard rumors that they can slightly degrade the quality of an audio signal, though I have also read that it doesn't matter because it is impossible for a human to discern the difference in audio quality. In my experience the fact that it so effectively kills any buzz/humm means that it is improving the quality of the audio signal. Here's the website for the manufacturer of the one I use:

http://www.xiteldirect.com/home.htm

They are also available at Radio Shack and from some other vendors but I only have experience with the Xitel model.