SB 7.4.24
tesam avirabhud vani
arupa megha-nihsvana
sannadayanti kakubhah
sadhunam abhayankari
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Then there appeared before them a transcendental sound vibration, emanating from a personality not visible to material eyes. The voice was as grave as the sound of a cloud, and it was very encouraging, driving away all fear.