Not like ranting - it's muttering/grumbling quietly. That kind of low-voiced complaining that you get when a sullen 14 year old slopes up to their room muttering "you can't make me." For me, the complaining to oneself element is important, even if you intend to be overheard there is still plausible deniability.

The internet suggests to me that it is northern, which would explain my familiarity, but it has also gained popularity from the Speaker's fondness for the phrase chuntering from a sedentary position to admonish MPs who have not been called to speak,