The title of the poem "Night Shift" gives some indication of what the unknown sound may be before we start reading. However, Plath's negations in the first stanza—it is "not a heart beating," nor is it "blood in the ears"—operate as synesthetic images; although we know the sound is not either of these things, the implication is that it could be, and that it must have the same sense of being part of the listener, "drumming" through her body.

This idea is prolonged in the second stanza; although it "came from outside," it is "native" to the suburbs in which it pounds. Like blood in the ears, it is the blood of the suburbs, the "thudding source" of it equated to the heart in a human body. The allusions to the heart and blood force the reader to view that source, in "Main Street's / Silver factory," differently, in terms of how it relates to the neighborhood. Even as we see the mechanics of what is making the noise—"hammers hoisted," "tonnage of metal and wood"—Plath uses the phrase "stunned in marrow," which recalls bone marrow and other natural, bodily elements. Although the noise may be mechanical, the association between it and the body and heart, as if it is the heart of the suburban body, continues.

Like the human heart, the noise of the machines simply continues as "indefatigable fact." The men in the factory are "tending" the machines; it is not as if the men themselves are making the sound, but as if they are almost servants of the machines themselves, "blunt" and "indefatigable": the machines cannot be worn out or killed. As the heart pumps blood indefatigably around the body, the machine pounds in the heart of suburbia, the lifeblood of the neighborhood.

The meaning and implications of Plath's poetry is usually profound.  "Night Shift" is no different.  There are many layers to this poem, starting from the sound in the first stanza.  This sound ends up constructing the entire narrative of the poem.  The speaker of the poem, perhaps someone like Plath herself, hears it.  The first stanza describes this sound as something that resonates:  "that clangor/Far off, not blood in the ears/Drumming up and fever."  It is a sound that impacts the evening air, something heard and cannot be blocked out.  Plath injects the "Native" to the "suburbs" of the poem's setting.  This sound is something intrinsic to the people living there.  The sound has become part of the environment:  "These stilled suburbs nobody/ Startled at it, though the sound/ Shook the ground with its pounding."  

The speaker of the poem investigates the source of the sound.  It is at this point where it becomes clear from where the sound is coming: 

Framed in windows of Main Street’s
Silver factory, immense

Hammers hoisted, wheels turning,
Stalled, let fall their vertical
Tonnage of metal and wood;
Stunned in marrow.

The presence of the factory and the lifeblood of the factory, the workers, is where the sound originates. The machinery that produces something undefined makes the sound, defining everyone who comes into contact with it.  The sound is ongoing, "indefatigable" and reflective of a condition that transcends being in the world. The sounds seems to define it.  The "Night Shift" aspect of the poem reflects how this sound and its source are embedded, ongoing and relentless in what it done and what is represented.  The individual who notices is almost dwindles in comparison to its magnitude and presence, one that underscores consciousness in the town and those in it.