But surely "Argh" is the sound of a sort of strangulated scream.
"Aargh" is the sound of a stabbing, or a falling off a cliff.
"Arr" is, I think, the noise you're looking for. It's the noise pirates make when they don't have anything better to do. "Arr, Jim Lad" = Pirate noise. "Aargh, Jim Lad" = sound of pirate falling off a cliff.