They had the correct answer.
Onomatopoeia: The formation of a word from the sound that word makes; A poetic structure of words that convey how something sounds.
I believe your answer is "moaning" and "groaning". When said, both sound as if you are doing the action of the word.
If you can pick more than two, than I might suggest "knell" as well, although it does not quite sound like the action. To knell is to ring solemnly.
I'll put this here for anyone still looking for the answer, or for anyone who needs confirmation.