A grumble is a sound or utterance that a human makes to his/herself, in a low, discontented way, usually in the form of a complaint. It can also be called a murmur or mutter.
When one grumbles, it usually occurs due to a negative experience.[1] An example could be: Jack's boss did not give him the promotion, so Jack grumbled to himself in frustration. Another example using "mutter": Frank muttered about the high taxes that he has to pay. Finally, an example using "murmur": The crowd murmured when the musical act suddenly stopped performing and everyone had to leave.