Ooh and aah aren't words, they're sounds (onomatopoeia). A sound is just a sequence of letters used for their phonological values.

You can spell the sound "ah" however you like: ah, ahh, aah, aahh, there's no wrong way to spell it.

If you write "the washing machine tringged when it finished", 'tring' is not a word, even though it's following the rules of English morphology, you could have written any sequence of letters that most faithfully reproduces the sound of the washing machine. You could have written katrigged or puh-tringged.