SynonymDefinitionAntonym.com - English Synonym Dictionary

A complaint uttered in a low and indistinct tone

A schwa that is incidental to the pronunciation of a consonant

A low continuous indistinct sound; often accompanied by movement of the lips without the production of articulate speech

An abnormal sound of the heart; sometimes a sign of abnormal function of the heart valves

Speak softly or indistinctly

Make complaining remarks or noises under one's breath