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 sound The sound made by a hungry stomach A complaint
Muttering
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.
Part of speech: noun
Definition: Something which is muttered