[Max Ripple]

These are lovely telly's,capable of very good performance even now!
From your description of the problem as not being affected by volume settings,it seems as though the output amplifier etc is at fault.
It's a stereo tv,so there are two seperate output amplifier chips (TD2040?)and a processor chip common to both channels.Any of these chips could be noisy,and it's useful to see if temperature affects the noise.
Do you have some freezer spray?If you apply this to the chips in turn and it reduces or increases the noise,then there is the problem?
I have the service manual too if you need more tech info.
Nick