All I know is, I hated it.
—
You’d hate it even more, if as part of your work day, you had to listen to it wearing headphones, while recording it, listening to the voice channels and transcribing the traffic.
here are various high frequency dog whistle hums, from #1 to #8. Do any resemble what you heard?
1:10 sec. ttl
https://www.youtube.com/watch?v=8oWaYF1MpGI
Sound Experiment - Dog Whistle