Say it out loud:
So fierce you whirr and pound, you drums—so shrill you bugles blow. (7)
How did you read that? Something like this, right?
So fierce you whirr and pound, you drums—so shrill you bug-les blow.
This is some flat out iambic heptameter here. (Don't worry, that just means that there are seven iambs [da-DUMs] in one line.) But this poem doesn't actually have a meter to it. It's written in free verse, which means it has no strict rhyme or meter. So why stick this iambic heptameter right in the middle of the poem? Perhaps Mr. Whitman wanted to bring us back to the feel of a marching band (da-DUM, da-DUM, da-DUM, da-DUM, da-DUM, da-DUM, da-DUM – you with us?). Or maybe he's making the point that we can never be totally free. With war on the brain, this is definitely a possibility.
It's also worth noticing that there is some structure to the poem: it's broken into three stanzas of seven lines each. If you want to get fancy, you can call these stanzas septets. Although the stanzas are all in free verse, they do have some sort of formula. They begin with the refrain (those repeated words we love so much), move into some of the effects the drums and bugles have on people, and end with a line (and the same word, actually) that emphasizes how forcefully the instruments are playing.