Humming vibrates and stimulates the soft palate (near the lower brain). It is a relaxing and self confirming sound.
This is a sidetrack from the central themes on the site, because only a few animals hum. Cats do something similar when they purr, frogs when they croak, and hens, camels, and elephants can make similar sounds. And with our human vocal cords, it's also a natural human ability.
Humming quietly is an easy and excellent way of keeping peace inside. It's particularly useful when everything outside is too noisy, busy and overpowering, like when walking through a crowd of bustling people.
It is ritualised by Buddhist monks when droning their texts. This is far simpler. There is no text!
It is also a way of regulating the breathing when savouring scents.