i beleive what you are experiencing is what is called a ground-loop problem. if you are using rca cables your signal will be unbalanced unless you use a transfer box to make it balanced. what i mean by that is, there is a box you can purchase (made by Ebtech $60-$80) that will allow you to run an unbalanced signal in one end and it will send out a balanced signal through the other end but you will have to have balanced cables hooked up to that end. now an unbalanced (TS) cable which is a mono cable like that of a guitar cable only have 2 wires that conect to the plug itself called (tip & Sleeve) tip connects to audio signal and sleeve connects to the ground. where as a balanced cable (TRS) have 3 wires that connect to the plug itself called (tip, ring, sleeve) tip to the left audio signal, ring to the right audio signal and sleeve to ground. all plugs on TS & TRS cables are 1/4 inch. hope this helps.
one more thing, Ebtech also makes a box called Hum Eliminater which works with TS & TRS cables. here is an example of what it does.
i had a keyboard that i borrowed from a friend a while back. when i hooked it up to my mackie i noticed that there was a hum comming from the keyboard. at first i thought it was the cables i was using. so i went out and bought new cables and the hum was still there. i tried everything to get rid of this anoying hum. i had heard about this box before but it never entered my mind at the time. then it finally hit me. so i ran out and bought the box, hooked it up and shazzam problem solved.
