The poem begins with a complicated sound:
The saddest noise, the sweetest noise,
The maddest noise that grows,—
The parallel structure in these lines makes it clear that this "noise" is all of these things at exactly the same time: heartbreaking, lovely, and liable to drive a person crazy (or a little crazy-sounding itself). Perhaps all these qualities even cause each other: the noise might be sad because it's sweet, sweet because it's maddening, maddening because it's sad.
What's more, this noise "grows." That might mean it starts quietly and gets louder. But it might also metaphorically suggest that this sound feels alive, like a flower shooting up from underground.
In the next two lines, the speaker reveals where this powerful, poignant, mysterious sound comes from:
The birds, they make it in the spring,
At night’s delicious close,
This noise, in other words, is the dawn chorus—the sound of birds welcoming a spring sunrise.
The speaker has framed this poem so that the "noise" of birdsong comes before the birds themselves appear—and it "grows" on its own, like an independent being. Readers who think of their own experiences of early-morning birdsong might see the truth in that: since one can't see all the singing birds, the sound does seem to grow straight out of the landscape.
Similarly, describing dawn not as the beginning of morning but as "night's delicious close," the speaker evokes the in-between feeling of the very edge of dawn, when it's neither quite morning nor quite night. And if it's "delicious," perhaps there's something special to be savored about that in-betweenness.
A "noise" that's at once the "saddest," "sweetest," and "maddest" fits right into this in-between landscape. There's an edge of tension, mystery, complexity here: no sound and no sight is one thing or the other. This bittersweet poem will take place in the borderlands between times and feelings.