iti mimamsatas tasya
brahmanah saha sunubhih
iti—thus; mimamsatah—while deliberating; tasya—his; brahmanah—of Brahma; saha—along with; sunubhih—his sons; bhagavan—the Personality of Godhead; yajna—Lord Visnu; purusah—the Supreme Person; jagarja—resounded; aga-indra—great mountain; sannibhah—like.
While Brahma was deliberating with his sons, the Supreme Personality of Godhead, Visnu, roared tumultuously like a great mountain.
It appears that great hills and mountains also have their roaring power because they are also living entities. The volume of the sound vibrated is in proportion to the size of the material body. While Brahma was guessing about the appearance of the Lordís incarnation as a boar, the Lord confirmed Brahmaís contemplation by roaring with His gorgeous voice.
Link to this page: https://prabhupadabooks.com/sb/3/13/23