SB 12.6.37

sūta uvāca
samāhitātmano brahman
 brahmaṇaḥ parameṣṭhinaḥ
hṛdy ākāśād abhūn nādo
 vṛtti-rodhād vibhāvyate
Word for word: 
sūtaḥ uvāca — Sūta Gosvāmī said; samāhita-ātmanaḥ — whose mind was perfectly fixed; brahman — O brāhmaṇa (Śaunaka); brahmaṇaḥ — of Lord Brahmā; parame-sthinaḥ — the most elevated of living beings; hṛdi — within the heart; ākāśāt — from out of the sky; abhūt — arose; nādaḥ — the transcendental subtle sound; vṛtti-rodhāt — by stopping the functioning (of the ears); vibhāvyate — is perceived.
Translation: 
Sūta Gosvāmī said: O brāhmaṇa, first the subtle vibration of transcendental sound appeared from the sky of the heart of the most elevated Lord Brahmā, whose mind was perfectly fixed in spiritual realization. One can perceive this subtle vibration when one stops all external hearing.
Purport: 

Because Śrīmad-Bhāgavatam is the supreme Vedic literature, the sages headed by Śaunaka desired to trace out its source.