muttering

nouns

a low continuous indistinct sound; often accompanied by movement of the lips without the production of articulate speech
murmur, murmuration, murmuring, mussitation, mutter, muttering
is derived from
mutter
a complaint uttered in a low and indistinct tone
grumble, grumbling, murmur, murmuring, mutter, muttering
is derived from
mutter