You might want to consider the way that Keats uses alliteration as part of the sound effect of this tremendous poem to create an image of the richness of autumn and how nature burgeons at this time of year. Consider the following example:
And fill all fruit with ripeness to the core;
To swell the gourd, and plump the hazel shells
With a sweet kernel; to set budding more...
The alliteration in "fill all fruit" emphasises the cornucopia of nature in this season, and words such as "swell" and "plump" are richly onomatopoeic in the way that they point towards images of bounty and plentiful harvest. Likewise, the repetition of the word "budding" in its various forms serves to reinforce this overwhelming impression of plenty. Nature's productive and fecund properties are therefore enacted and supported by the sound effects that Keats uses in this excellent poem.