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
Grumbling
Part of speech: verb
Definition: To make a low, growling or rumbling noise, like a hungry stomach or certain animals.
Part of speech: noun
Definition: Complaining.Rumbling.
Example sentence: A lot of times, sets are not such fun places to be, and especially television sets, because everybody works so hard, and everybody's kind of grumbling about getting up at 3 o'clock in the morning and not getting home until midnight.