I'm thinking who ever is doing it is using several Ultra high frequency emitters that are supposed to be emitting the same exact frequency sound, but they don't. They are probably drifting off slightly, probably because of temperature and other factors, and causing beat noises that can be heard and that change as the emitters drift in different but close frequencies. The beat sounds are probably not intended and are a side effect.
I'd rule out Russia because they probably have good enough tech that they wouldn't be making that mistake.
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