We still don't know what (if any) influence the squelch control has on it.
If this radio was made in 1978 it could have the original "TA7222" audio power chip, and NOT the one with the letter "A" at the end of the part number.
I remember some odd noises coming from that original-type chip 35 or more years ago. Pretty sure replacing it with the "A" fixed this.
And if the chip has that letter "A" after the "TA7222" markings, that's probably not it.
It's not just the miles, it's the years, too.
We test for noisy electrolytic caps by bridging a new cap across each of the caps C112 (4.7uf) and and C115 (22uf). If the noise goes away or gets reduced in a big way with the new cap touching the foil pads, that one is causing the noise. C114 (47uf) is a possibility, but I can't remember seeing that one cause this fault.
73