What Esperanto verb form is used for sounds in comics when the sound cannot be described easily by spelling it out? An example is English sigh, German seufz (note the pure verb stem being used here, a verb form termed Erikative in honour of the German translator Erika Fuchs).
There’s a very nice Esperanto translation of an Italian comic called Rat-Man. In that it seems to use the root word with an apostrophised ending. I guess that means it is using a noun instead of the verb. I think it works well and it sounds very similar to what you describe in German. For example: