This is fascinating and I am definitely adding it to my research and practices. I really enjoy the hamsa mantra where ha and sa represent the in and out of the life force and the m in between breaths represents Shiva, or the higher self, the big "I". If one is to hold the breath right there, for just a moment, you can achieve union. So there are two different kinds of hums, one internal and one external. I actually hit a particular resonance yesterday while doing the isha kriya, near the end when you let out the ah sound 7 times. The first one was resonating so well that I felt myself expanding and an experience very similar to the toad came over me and I felt like if I could just hold that note for a moment longer, it would have broken me through. I believe in this 100%.