Attached is a composite of 4 short clips of both VHF and HF comms with the "Mystery Tone" in question.
The clip starts off with United 36 followed by United 41, then Dynasty 3. I selected these clips since you can clearly hear the "tone".
For the last track, I isolated the "tone" and looped it for 5 seconds.
There is 3 seconds of silence between the tracks.
I've heard this for years. Is there a technical name for it? What creates it?