This hooty signal popped up all over HF on practically the same day the Mexican Army's loud clear-voice "radiogrammas" vanished from the air. Since nothing much else changed about this traffic, which was heard constantly in the southwestern US, we decided it was almost certainly the same people. It then vanished as abruptly as it had appeared, replaced by the ALE nets with the funny names. These, too, now appear dead.
This stuff sounds quite a lot like the old Harris Analog Voice Security (AVS). It strongly appears that the resulting waveform comes out in several narrow, very ringy bands, as we see clearly from the spectrogram. We also see how quite a bit of inflection makes it through the process, allowing the brain to pull out an occasional Spanish word. Never underestimate "wetware" decoding!
The amplitude graph at the top of the first picture is interesting in that the received waveform is completely symmetrical. This is not a characteristic of natural human speech, and it further explains the mechanical sound of this mode.
The highly averaged frequency plot below shows the precise spacing of the inverted bands, not to mention their extreme peakiness. This dramatically shows the reason for the hooty sound.
This all happened a number of years ago, before Mexico was quite as dangerous as it is now. The traffic that had been thus secured was VERY mundane discussion of recruiting "guarnacions" and toilet paper requisitions. An army travels on something else besides its stomach.
All plots made with GRAM.EXE.