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.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
तेषामाविरभूद्वाणी अरूपा मेघनिःस्वना । सन्नादयन्ती ककुभः साधूनामभयङ्करी
teṣāmāvirabhūdvāṇī arūpā meghaniḥsvanā | sannādayantī kakubhaḥ sādhūnāmabhayaṅkarī
SB 7.4.24