Humming quietly to yourself is a self confirming sound.
This is a sidetrack from the central theme of the essay, because i only use it occasionally, it's not a necessity for survival, and only a few animals do anything like this. I believe cats do something similar when they purr, frogs when they croak, and hens and camels can make similar sounds.
The possibility comes naturally with our human body, and our human vocal chords. I find it is ritualised by Buddhist monks when droning their texts. This is far simpler. No text.
I find humming quietly to myself, a useful way of keeping peace with myself particularly when everything outside is too noisy, busy and overpowering, like when walking through a crowd of bustling people. At these times i'm not interested in scenting and my mouth is closed.
Extra Humming Games
At birth, when the breathing starts, we start smelling and we start making noises with our mouth.
If first you sense taste, from the lips and mouth down into the body; then with the mouth closed, sense the smells in and above the roof and sides of the mouth. Then feel how the soft palate is the centre of this area - and notice how when you start humming and it vibrates this central area, (and also the top and back of the throat). This stimulates the lower brain. ... and experts in the field of the lower brain could tell us more on this ...
Sometimes you may feel the humming, vibrating in your belly, i don't know the next step on that route. - You can also focus this humming and vibrating anywhere stiff in the body. Quite generally it seems healthy to allow yourself to make any sort of vocal noise you want, to vibrate and massage yourself inwardly.