Humming quietly to yourself is a self confirming sound.
This is a sidetrack from the central theme of the essay, because humming is only rarely a necessity for animals survival, and only a few animals do anything like this. I believe cats do something similar when they purr, frogs when they croak, hens, camels, and elephants can make similar sounds. And with our human vocal cords, it's also a natural human ability.
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.
I find it is ritualised by Buddhist monks when droning their texts. This is far simpler. There is no text!
It vibrates and stimulates the soft palate (near the lower brain). It is also a way of regulating the breathing when savouring scents.