There's a possibility that you may be experiencing problems with the audio output IC -IC404 - All serial numbers produced until July 2003 might use one of these defective IC's. So if your set was manufactured before this date, that is probably the culprit. If not, and you have it connected to a cable box that has its own volume control, it could be that its volume is turned down and by turning up the TV's volume too high that you are getting this distortion. Hope that helps.
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