Ok! Nice work. We can rule out RF gain for now...
The steps taken - resulted in knowing the RX was not ADDING to the noise, then focus on the Audio Amp chip area.
The process of "Breaking connection" to any portion of the receive, Mic or PA amps - you have isolated it to the Audio Amp itself. So if you need to, go ahead a do the same for the Mic and PA sections - to be sure.
Then once we know NONE of this noise is arriving to the Audio Amp section - focus on that section as being the culprit.
Look for these kinds of caps - I call them Mylars, Green Chiclets, but "PC Crowd" calls then Polystyrenes...
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So if you have any of those in your radio, you already may want to replace these with standard Disc Caps - the beige or brown types - use the same values. These "styrene's" have been known to have an affinity for moisture and although a much higher breakdown voltage than their Disc counterparts - you don't have those kinds of hazardous voltages running thru those areas, or thru the radio even. (Sorry for the Preposition) So they are CHEAPER than their Disc counterparts and work well for their package size and work well in audio bandpass filters - but when the fail - they fail - miserably.
When you do some more research, you'll see that Uniden and other radio makers, may use these but in very rare instances - because of a need to fill that they couldn't drop in another more reliable type to meet the specs. But as with experience, you'll appreciate certain brands of radios sticking to quality more than cheaper easy to fix parts that when it comes to set it and forget it - makes you wonder why these Styrenes ever existed.
So again, we will be looking for those Green Chiclets and also into a possible recapping of Electrolytic caps (Grey ones rated 10V are problematic too - they just "kill" the circuit by shorting out when they've dried out.
