It’s a continuous, constant tone humming sound, very clear.
The author says:
The most interesting thing: This sound is a “Mi” note…
Do-Re-Mi-Fa-Sol-La-Si.
This one is a “Mi” (E) tone; and I’m sure it has 528Hz waves…
(between 1-2 seconds and 4-5 seconds of the video is when the sound is higher…)
