I doubt it's what's going on with most TVs (especially since LCD TVs don't seem to have this problem), but my old, cheap TV tuner card emits a 15.9kHz squeal on

I doubt it's what's going on with most TVs (especially since LCD TVs don't seem to have this problem), but my old, cheap TV tuner card emits a 15.9kHz squeal on its line out, so it's not impossible.