Mmmm is something that people (English-speakers?) say when something tastes good, seems appetizing, or is interesting or arousing or something. It's not exactly a word, but it's not an onomatopoeia.

"Aaaarfgfhfhrhhhh" would be an example of someone typing frustratedly into the keyboard (or mashing their keyboard) because something didn't go their way. It's an arrangement of letters to express frustration. It isn't even necessarily a sound that anyone would vocalize. So it's an arrangement of letters that conveys a thought but isn't a word.