@HYBRID_BEING our policy for signs and onomatopoeia is simply just to leave them be. the only times we translate those are when it's relevant to the plot (i.e. without it being translated, the reader would be left confused). specifically for onomatopoeia, japanese has what amounts to an entire language. while it's possible to translate these (to a degree), often there's no direct translation, so you end up with some weird string of kana, with some form of somewhat-relevant english slapped on top of it. usually, the original japanese onomatopoeia serves its own purpose without translation: e.g. you can see something happening in the panel, and there's japanese fx text that you don't need a translation for to understand, contextually.
i think the blank spots you're referring to were cleaned by the staff cleaning the chapter, but weren't addressed by the translator, partially for the reasons i stated above, and partially to get the chapter out as quick as possible. i don't think it's a matter of being unable to translate. onomatopoeia in japanese is just particularly complex, often so much so that a native speaker would, rather than try to translate, just tell you "it's just a noise that means XYZ".