So, this has been a painful journey.? I have dim hope that it is nearing an end, but as is often the case, one can never be sure that an intermittent is fixed, but one can be sure when it has NOT been fixed.? In this case, I got about a week of operation before the fault returned.
I looked at many possible culprits focused mostly on intermittent open conditions (in a via or component).? The speaker amplifier turned out to be a dead-end, and there was nothing wrong with that circuit.? After many disassembly-reassembly cycles, I finally found a cogent result: IC2551-B ended up being the stage where the "noise" was being introduced.? This is a filter circuit used for all modulation modes OTHER than WBFM, AM, and the TV mode (a more careful examination of the behavior of the different modes might well have pointed to this part of the circuit much sooner -- lesson learned).
I replaced this IC with a close alternate (TL972IPWR).? So far, the result is good.? Until, it isn't.
I don't know if this will help anyone, but hopefully is does.? If anything, it suggests that these "strange" faults can be repaired, but one should be very methodical and thorough.? Learning the details of how the circuit works is also an important task to accomplish.
Cheers
Joe KE0FF