Each sound is a sound of contentment (4)
I believe the answer is:
purr
'sound of contentment' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for purr that I've seen before include "Noise of a contented feline" , "Sound of cat (or engine)" , "Low vibratory sound" , "Appreciative feline sound" , "Cat's noise" .)