In active water flow water jets constantly capture air bubbles. They immediately pop up, forming on the surface of a stream of bubbles, which now burst. Faint sounds that occur at the same time, mix and create the kind of noise that we call murmur.
Sounds are vibrations of an elastic medium, and that this environment is beginning to waver, we first have to shrink or stretch. The rapid flow of water around the bend or knot, turns into a seething mixture of air and water, and when compressed water air unleashed, we hear that the most pleasant murmur.