Canto 1 - Boyhood
Bāla-kāṇḍa
Chapter 24: The History of Sarayū
Text 1.24.5

ततः शुश्राव वै शब्दमतिसंरंभवर्धितम्।
मध्यमागम्यतोयस्य सह रामः कनीयसा॥

tataḥ śuśrāva vai śabdam atisaṁraṁbha-vardhitam
madhyam
āgamya toyasya saha rāmaḥ kanīyasā

tataḥ śuśrāva vai = heard; śabdam = a sound; atisaṁraṁbha-vardhitam = loudened by the rapid splashing of waves; madhyam = the middle; āgamya = when they reached; toyasya = of the waters; saha = and; rāmaḥ = Rāma; kanīyasā = his younger brother.

When they reached the middle of the waters, Rāma and His younger brother heard a sound loudened by the rapid splashing of waves.

Or is the sound caused by something else? This was Lord Rāma’s inquiry.