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You didn't mention any odd smells, but if it's a screeching sound that sort of varies in volume, and you also smell ozone coming from the set, it's probably HV arcing. As Kaye-Halbert TV suggests, it might just be that the 2nd anode lead is loose. If that's not it, try operating the set in the dark with all of the room lights off and see if you can spot the location of the electrical arcs.
It could also be a way-off horizontal oscillator as others have suggested, but flyback squeal has a different/distinctive sound that should identify it.
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