Murmur

mruczenie

(low sounds or speech)

pomruk

(low sounds or speech)

szmer

(heart sound phenomenon)

bąkać

(to speak or make low, indistinguishable noise; to mumble)

bąknąć

(to speak or make low, indistinguishable noise; to mumble)

mamrotać

(to say something indistinctly, not necessarily in a low voice)

sarkać

(to complain or express discontent)

sarknąć

(to complain or express discontent)