Ya, I can tell there's a higher pitched sound along with what I was hearing. It reminds me of those electronic dog whistles, or those new sonic things dentists use. There's some sound that you can hear, but most of it is more of a feeling than a sound.
These were short bursts of sound.







, and just now recall one a few years ago that came gliding across the road late one night & almost took me off my Harley!! 





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