Vijnana Bhairava Tantra · Dharana 15 · Verse 38
River sound or inner hum
Close the ears and listen for the inner sound that is already there: a continuous hum, made by nothing striking anything, rushing on like a river. If it is too faint at first, sit by running water and let that unbroken sound gather the mind. The point is to give attention a single, unbroken sound, so that it can settle.
Older line
One who is absorbed in the unstruck sound, heard within when the ears are closed, unbroken and rushing like a river, attains the supreme Brahman.