We run into this is some bars where their TV system has obviously got wiring switched somewhere. A simple inexpensive solution is a ground lift (the little plugs that allow you to plug a 3 prong grounded plug into a non-grounded outlet). Not sure if it will solve your problem, but this is my first step anytime my system buzzes/hums when I hook into the TVs and it quits when I unplug the TV.
Wayne
MTUSUPPORT ADDED: Wayne, we have also found that these are needed with the laptops that we are selling. We have found t here is a grounding issue somwhere within them, that will send some noise out to the stereo. The fix is as you stated a Ground Lift Plug from Radioshack. The part Number is: 61-2720. It can be found at: http://www.radioshack.com/product.as...5Fid=61%2D2720