Mutter
Part of speech: noun
Definition: A repressed or obscure utterance; an instance of muttering. (in Indian restaurants) peas
Part of speech: verb
Definition: To utter words, especially complaints or angry expressions, indistinctly or with a low voice and lips partly closed; to say under one's breath. To speak softly and incoherently, or with imperfect articulations. To make a sound with a low, rumbling noise.
Grumble
Part of speech: verb
Definition: To make a low, growling or rumbling noise, like a hungry stomach or certain animals.To complain.To utter in a grumbling fashion.
Part of speech: noun
Definition: A low thundering, rumbling or growling soundThe sound made by a hungry stomachA complaint