Those were a solid set.
As a general rule audio can be broken down to stage easily.
1) hissing but has audio. Look at sound IF
2) buzzing or ticking. Go to sound detector ( quad, disc.).
3) 60 cycle hum. Output or power supply.
Detector is most common. Flag an alignment tool & rock the
detector coil slug back & forth one turn. ( this may use a square slug.
If it clears note how far you turned it took CW or CCW. That way
you can go back if you have to. Save that note inside the set. If it drifts off again something has failed.
73 Zeno
LFOD !