croon

Short definition from Wiktionary

Verb

1. 1
   To hum or sing (a song or tune), or to speak (words), softly in a low pitch or in a sentimental manner; specifically, to sing (a popular song) in a low, mellow voice.
2. 2
   To soothe (a person or an animal) by singing softly.
3. 3
   To hum or sing, or to speak, softly in a low pitch or in a sentimental manner; specifically, to sing a popular song in a low, mellow voice.
4. 4
   To lament, to moan.
5. 5
   To make a continuous hollow low-pitched moan, as of cattle; to bellow, to low.
6. 6
   Of a large bell: to make a low-pitched sound.

Noun

1. 1
   A soft, low-pitched sound; specifically, a soft or sentimental hum, song, or tune.
2. 2
   A continuous hollow low-pitched moan, as of cattle; a bellow.
3. 3
   The low-pitched sound of a large bell.