to utter words indistinctly or in a low tone, often as if talking to oneself; murmur.

to complain murmuringly; grumble.

to make a low, rumbling sound.

to utter indistinctly or in a low tone

to mutter complaints.

the act or utterance of a person who mutters.

noun

complainer , crab , faultfinder , growler , grumbler , grump , murmurer , whiner