Ground Potential Hum

[vts1134]

I have a combo set that I use frequently that has an issue with the audio. When I connect the TV and radio chassis electrical grounds together I get a buzz in my speaker. The sound is definitely higher than 60hz. I can use an audio isolation transformer to get rid of the problem, but I want to understand what's happening to cause the problem, and if it can be fixed without resorting to the isolation transformer.