Purr

google

ref

early 17th century: imitative.

wiktionary

ref

Onomatopoeic. 

etymonline

ref

purr (v.)

"uttering a low, murmuring sound expressive of satisfaction or pleasure, as a cat," 1610s, of imitative origin. Related: Purred; purring. As a noun, "sound made by a purring cat," from c. 1600. A similar imitative word, curr "make a low murmuring sound, purr" (1670s), was used of doves and cats.