The Wikipedia article, “Checked and free vowels“, refers to the following as “onomatopoeias”:

yeah /jæ/; eh /ɛ/; duh, huh, uh, uh-uh, and uh-huh with /ʌ/.

If an onomatopoeia is properly defined as a word that imitates a sound which exists independent of that word, such as crash, burp, whir, or gulp, then none of the listed words is an onomatopoeia. Yeah (which is not pronounced /jæ/ in General American but /ˈjeə/) is simply a particle equivalent to yes or yea (whatever part of speech one considers those to be). Eh (which likewise is not pronounced /ɛ/ in General American but /ˈeɪ/) may be an interrogative (c.f. Ger. nicht wahr?, and oder?, Fr. non?), or it may serve merely as a meaningless place-holder like duh, uh, um, etc.  Huh is usually an interjection expressing puzzlement, surprise, or doubt. It can also take the place of interrogative eh.  Uh-uh and uh-huh mean, respectively, no and yes, and are therefore in the same category as yeah. None of them imitates a sound that exists independent of the word itself, so none is an onomatopoeia. (Arguably, huh is an onomatopoeia for a kind of snort:  cf. the onomatopoeia humph!; but if so, it is the only one on the list.)

The standard of Wikipedia’s linguistic articles is usually higher than this. An editor who understands the concept of checked and free vowels (I don’t) should revise the passage.

 Originally posted on the “talk” page for the wikipedia article.

Note that I would have written hunh for huhunh-hunh for uh-huh, etc., to reflect the nasalization of the vowels in those words, but didn’t want to vary from the spelling used in the article already.  I observe, moreover, that the spell-checker in Google Chrome thinks my spellings are wrong, for whatever that may be worth.  When I was a boy growing up in Virginia, we distinguished between the interjections huh /hʌːʰ/, expressing strong disbelief, and hunh /hʌ̃ː/, which expressed puzzlement, surprise, or mild doubt.