Purr (verb)

1. To make a low, vibrating sound, typically one of pleasure or contentment, as a cat does.
2. To make a low continuous humming sound, as of a machine running smoothly.

Origin:

Of imitative origin.

Examples:

1. The cat was purring contentedly as it lay on her lap.
2. The car engine began to purr as he turned the key.
3. The kitten was purring softly as it slept.
4. The purring sound of the cat lulled her to sleep.
5. The purring of the engine indicated that it was running smoothly.