I heard a Fly buzz – when I died –
The Stillness in the Room
Was like the Stillness in the Air –
Between the Heaves of Storm –
The Eyes around – had wrung them dry –
And Breaths were gathering firm
For that last Onset – when the King
The fly has a huge presence throughout, which is amplified here.  Death engenders a distinctive atmosphere — silence and thoughtfulness as well as grief and tears.  The repetition of “Stilless” suggests quiet and serenity, but this is undermined by the fly. 
 The reader may wonder if this irritating insect signifies the speaker’s feelings about herself; the littleness of individuals, even dying ones, in the face of momentous death. 
 There are two contrasting effects, that of quiet and serenity and, in contrast, the momentous and violent “Heaves of Storm”; the latter we presume representing death. 
 The  alliterative , sibilant “s"s imitate the sound of the buzzing fly. And yet this mutates into the storm — and so the boundary between the fly’s presence on earth and the speaker’s death is blurred.
I heard a Fly buzz 465