One was hitting a rock with another rock and humming an incomprehensible baby hum.
The air was alive with the spirit of the place, fragrant and humid, and humming with energy.
When the economy is humming along nicely, few people consider the amount of trust involved in every transaction.
late 14c., hommen "make a murmuring sound to cover embarrassment," later hummen "to buzz, drone" (early 15c.), probably of imitative origin. Sense of "sing with closed lips" is first attested late 15c.; that of "be busy and active" is 1884, perhaps on analogy of a beehive. Related: Hummed; humming. Humming-bird (1630s) so called from sound made by the rapid vibration of its wings.
There is a curious bird to see to, called a humming bird, no bigger then a great Beetle. [Thomas Morton, "New English Canaan," 1637]
mid-15c., from hum (v.).
A low, continuous murmur blended of many sounds.