What is the synonym for grumble?

Here are some synonyms for "grumble", depending on the context:

General Complaints:

* Mutter

* Complain

* Whine

* Moan

* Grouch

* Bellyache

* Gripe

* Kvetch (informal, Jewish English)

Low, Rumbling Sounds:

* Rumble

* Growl

* Hum

* Buzz

Informal/Slang:

* Beef

* Whinge

The best synonym for you will depend on the specific situation and the tone you want to convey.