Only humming a little, the quiet sound in his head.
The "quiet sound" may mean
- the sound of "his" humming
- that the humming was produced by "the quiet sound" in his head
- that the two things are separate and possibly unrelated
Poetry is often written in a very dense style, wherein each line layers much meaning over few words, so it could even mean "all of the above."
Of the three things listed, however, if I had to pick a single one I would favor the second choice. The only thing that looks like a subject in that sentence (not that it needs a subject, this being poetry), is "the quiet sound". Let's rearrange that sentence and put the subject first:
The quiet sound in his head [was] only humming a little.
Now its meaning becomes a little clearer, especially if we furnish the missing copula (verb of being).
In the original order, the line is an example of anastrophe, which is a rhetorical device which means
a departure from the conventional subject, verb, object word order. [Ward Farnsworth, Classical English Rhetoric]
Farnsworth goes on to say that "the unexpected order of words calls attention to them ... [and this ordering] mildly violates the reader's expectations."
It appears to be the poet's intention here to place greater emphasis on the slight humming, and to reveal the topic of the sentence only later after the object had been introduced.
Note that removal of the copula "was" is an example of the rhetorical device called ellipsis, which is the removal of words for dramatic effect. Poetry is all about dramatic effect, so one should not be surprised to find more than one literary technique to be used in each and every line.