Sound of contentment (4)
I believe the answer is:
purr
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">sound of contentment</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)