Synonyms for Muttering (same or very similar meaning)
WordNet sense 1 (a low continuous indistinct sound; often accompanied by movement of the lips without the production of articulate speech):
murmur, murmuring, mussitation, mutter, murmurationWordNet sense 2 (a complaint uttered in a low and indistinct tone):
grumble, grumblingFrom the ODE community, based on WordNetadd/edit