To Mutter

1. To utter words, especially complaints or angry expressions, indistinctly or with a low voice and lips partly closed; to say under one's breath.

2. To speak softly and incoherently, or with imperfect articulations.

3. To make a sound with a low, rumbling noise.

(speak under one's breath): growl, grumble, mumble

(speak incoherently): babble, mumble, murmur, ramble, stutter

(make a low sound): growl, putter, rumble