[John123]

Maybe have a go at tuning in the tuner to a blank screen from an RF modulator/ VCR (VCR set to its A/V channel). Perhaps that might rule-out the tuner/ IF stage as a source of the noise?

These sets often feature a plethora of DIN A/V, Tape, Amplifier in's and out's. Try tapping the audio directly from one of these (or routing audio through them). It should be possible to tap the audio from a tuned RF channel from one of theses sockets (or the SCART). That would be quite telling in itself, perhaps narrowing things down to the power amplifier stages.

Reseat all board interconnects and edge-type connectors by plugging/ unplugging several times. Inspect solder-side for dry joints.