While Brahma was deliberating with his sons, the Supreme Personality of Godhead, Visnu, roared tumultuously like a great mountain

Expressions researched:
"While Brahma was deliberating with his sons, the Supreme Personality of Godhead, Visnu, roared tumultuously like a great mountain"

SB Canto 3

While Brahmā was deliberating with his sons, the Supreme Personality of Godhead, Viṣṇu, roared tumultuously like a great mountain.

SB 3.13.23, Translation and Purport:

While Brahmā was deliberating with his sons, the Supreme Personality of Godhead, Viṣṇu, 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 Brahmā was guessing about the appearance of the Lord's incarnation as a boar, the Lord confirmed Brahmā's contemplation by roaring with His gorgeous voice.