Fitsmart Fat Burner Dragons Den Uk :...
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No best answer has yet been selected by chazza. Once a best answer has been selected, it will be shown here.
For more on marking an answer as the "Best Answer", please visit our FAQ.It sounds very much as if your picture signal is encroaching on your sound channel. This would explain why the noise increases when some strong addition to the picture, e.g. a white title, suddenly appears.
When you say that the problem is cured when you knock the station off-tune you are probably, in fact, tuning Channel 4 in properly, with the vision and sound in their proper place.
If your manual tells you how to lock in this 'new' tuning (it should do so when explaining how to tune channels manually) then it should hold that tuning when you switch off and on again.