I would but I need him to be able to verbalize the sound. I could do the arghhtgggtaarrgrhh but it just doesn't look right.
Why? Is there a reason you need to spell it out?
Chances are, even if you get it perfect, nobody's going to read it like a Wookie cry unless you spell out that information, in which case you don't need to write the sound down.
Personally, if I came across "arghhtgggtaarrgrhh," I would stop at "arghh" and assume he was expressing distress about something. If I read, "'Arghhtgggtaarrgrhh,' he yelled, sounding like Chewbacca from
Star Wars," I would think, "Oh, that's what those letters sound like," after the fact. If I read, "He gave a cry like an annoyed Wookie," I know exactly what that sounds like and you didn't have to pull me out of the story to do it.