In his essay, ‘The ways of the possible: a textual analysis of “gifts of rain” by Seamus Heaney,’ Adolphe Haberer analyses instances of chiasmus in the poem at the phonological level.
I will focus on one line (the first) for the sake of time, and break it up as he does phonetically to reveal the pattern beneath:
“Cloudburst and steady downpour now”
A: kl aʊ d
B: b st
B: st d
A: d aʊ n
or aʊ st st aʊ.