Commentary Par XX 16-18

The silence of the souls, having left off their singing (which Dante could not hold in mind [[Par XX 12]]), begun when the Eagle had ceased its speech, gives way to what seems to be the rumbling sound of a river, giving evidence of the profusion of its lofty source (it will be the voice of the Eagle, rumbling like an organ pipe filling with new air).  This tercet marks the beginning of the first of the two central elements of the canto, a presentation of the souls that make up the eye of the Eagle (vv. 16-78); the second, the Eagle's explanation of the presence in Paradise of those who certainly appear to be pagans, runs through vv. 79-129.