If only we had a physicist in our midst.
It sounds to me (a non-physicist) like a binaural beating phenomenon. A sine wave should be easy enough to produce with freeware of your choice so you might try obvious mathematical approaches. IIRC the third beat to a psycho-acoustical binaural frequency pair is the difference between the two so 337.9 or 272.5 Hz would be possibilities, or better yet set up the 4th speaker directly fed by the active sine source and sweep the frequency. You might find something even cooler than the original “blue” candidate.
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