murmur

Short definition from Wiktionary

Noun

1. 1
   Any low, indistinct sound, like that of running water.
2. 2
   Soft indistinct speech.
3. 3
   The sound made by any condition which produces a noisy, or turbulent, flow of blood through the heart.
4. 4
   A muttered complaint or protest; the expression of dissatisfaction in a low muttering voice; any expression of complaint or discontent.

Verb

1. 1
   To grumble; to complain in a low, muttering voice, or express discontent at or against someone or something.
2. 2
   To speak or make low, indistinguishable noise; to mumble, mutter.
3. 3
   To say (something) indistinctly, to mutter.