This kind of reminds me of some of Jonathan Goldman's work on Healing Sounds. In one of his books (Shifting Frequencies?) he describes how the rhythmic humming of a truck sent him on a journey outside of space and time where he met some tribal elders who were using the same kind of frequencies for journey work. Anecdotal at best, I can easily see how his credibility might lie more in the direction of woo and New Age BS - especially considering how his tuning forks are decidedly overpriced - but he is pretty accomplished in the field of vocal overtone singing.