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Before we answer the question, let's define what an Onomatopoeia is:

An Onomatopoeia is a word that is used to resemble or mimic the source of the sound it describes. These are great literary devices, especially in poetry, to convey realism in depicting a subject or a character. 

Now there are different types of Onomatopoeia. 

They are:

Direct- which resembles the exact sound, like zoom, swish, or bang.

Associated - means the words that resembles a sound associated with it. Like thud, cuckoo, or splatter.

Exemplary - means the onomatopoeia that describes the physical movement or work used by a speaker when describing a word that matches its meaning. Examples are: mumble, and dart

In the poem, we can say that the following are onomatopoeic:

1. Mewling (Exemplary)
2. Whining (E)
3. Sighing (E)
4. Whistles (A)

These are the only onomatopoeic words in this poem. 

4 3 4