Sound of contentment (4)
I believe the answer is:
purr
'sound of contentment' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for purr that I've seen before include "Soft noise of engine" , "Sound of cat (or engine)" , "Make contented sound like cat" , "Cat's coo" , "Low vibratory sound" .)