WordNet sense 1 (an abnormal sound of the heart; sometimes a sign of abnormal function of the heart valves):

heart murmur, cardiac murmur

WordNet sense 2 (a low continuous indistinct sound; often accompanied by movement of the lips without the production of articulate speech):

murmuring, mussitation, mutter, muttering, murmuration

WordNet sense 3 (a complaint uttered in a low and indistinct tone):

grumble, grumbling

WordNet sense 4 (a schwa that is incidental to the pronunciation of a consonant):

murmur vowel

WordNet sense 5 (make complaining remarks or noises under one's breath):

croak, gnarl