Here is a excellant paper on hum and buzz in a unbalanced interconnect system; google jensen an-004;written by Bill Whitlock.
Ground Loop
What is a ground loop? I understand that they can be caused by loose connections between components. What else can cause them? What are the effects? I have been told that humming from my power amplifier can be a symptom of a ground loop. True/False? Can ground loops cause damage to equpment?