I would ask the same question and give the same answer but, additionally, older TV sets used to suffer from 'sound on vision' which meant that when the picture got too bright or too much contrast (like text subtitles on the screen) the TV started to distort the sound, humming etc. It too was usually caused by poor tuning so turning off the afc might cure it. Modern TV's have inbuilt selection of channel frequencies, it might just be that you are recieving signals from another transmitter which is using an adjacent frequency and your afc is fighting between the two and can't decide which it should be 'locking on to'.