Creeper priming with procedures

Hello I am trying to make a mod to change the creeper priming/explosion sounds, so far I have managed to change the explosion sound by using the entity about to grief the world trigger and play the custom sound, but I haven't found a trigger to activate on creeper priming(hissing?) to play a different sound. I tried using on target aquire or something like that but that did not trigger the way I wanted. Is there some trigger I missed, or maybe a better way to do it?