Separating Video and Audio in Split Carrier Sets

[vts1134]

I've got a set on the bench with an odd audio problem. After recapping the audio had a lot of buzzing along with the audio track. I suspected the problem was video feeding into the audio circuit. I've plugged enough RCA video cables into audio ports doing installations blind to know what it sounds like. My suspicions were confirmed when I adjusted my modulator's video level output to minimum and the audio came through loud and clear. So my question is how is video and audio separated in a split carrier set? How can video be "infecting" my audio?