Whilst you are surely on the money saying that there is a good chance that an AGC circuit is actively involved in this issue, I think we have to once and for all get away from the idea that this an adative Signal+Noise on the audio problem. Surely the Squelch experiment clearly demonstrates that there is no constantly present unusual noise source, but instead something more akin to an AGC oscillation/instability and/or unusual modulation phenomena somewhere in the signal chain that is induced as a result of some mechanical phenomena near to the actual antenna. The ADF wire removal is going to provide to some very good data.
Not sure that GY was saying that there was an "unusual noise source" involved.
I read it that he was puzling over why the max gain (no signal at all) audio did not ramp up the circuit self noise to a point where it at least partially resembled the problem noise.
HF did mention a "chop" to it which I suspect is related to the AGC misbehaviour.
The reason the AGC does not misbehave with no signal is simply that the interlnal self noise is constant, not modulated by whatever is causing the problem.
Even with almost perfect electronics the external background noise would be non directional.
Misbehaving (especially oscilating) agc can cause all sort of weird effects if it causes levels at the detector way beyond design targets.
Historical note: Bell labs used liquid helium cooled amplifiers (and a host of other tricks, google "big ear") for early satelite groundsations, still had noise and eventually discovered the "big bang" cosmic background radiation.
I saw GY.s response after I posted this, concise wins the race
BTW: Also heartily agree on thanks to moderator, I have no problem with ignorance (have plenty of my own) but the **** posts were not that...