Shorted turn test
One possible effect is that the ADF aerial in conjunction with the fuselage makes a shorted turn around the comms antenna(s).
I have just run a very crude simulation. I used a small transistor radio receiver on 107MHz (near to the aircraft band) and held a shorted turn of wire so it hung around the radio.
As I rotated the loop, keeping its axis horizontal, it was possible to see changes in signal strength on the tunng LED. There was attenuation of signal and it did vary as the loop was rotated. It appears that a shorted turn around an antenna does cause attenuation of the signal and that this is directional.
When the shorted turn was in the orientation that gave most attenuation the noise came up, as might be expected. The signal drops, the agc increases the gain, and the noise appears to increase. This noise is simply the receiver internal noise in the early stages and the first mixer.
This is not a full explanation of what is happening but it may be contributory. I hope that HF will listen to my requests and at least do a ground test with and without the ADF aerial fitted, in amongst his busy schedule of flights.
If this gets a result then we may be able to add further aspects to explain the remaining symptoms.