Sound of braying (6)
(Other definitions for hee-haw that I've seen before include "Cry of donkey" , "Bray" , "Braying sound" , "Cry of a mule" .)
(Other definitions for heehaw that I've seen before include "Ass's bray" , "Donkey's bray" , "Sound from the stable?" , "Cry of donkey" , "Horse-laugh" .)