grumble

To grumble means to complain or make a low, rumbling sound, often because you're unhappy about something.

When people grumble, they usually do it quietly to themselves, often with a bit of a growl. For example, if someone's been waiting in line for hours and finally gets on the plane only to find out there are no more seats available, they might grumble under their breath as they try to figure out what to do next. You might hear people grumbling about traffic or weather too – it's a way of expressing frustration without really saying much at all.