what it mean if someone writes "grumble, grumble, grumble"?

[Amy]

what does it mean if i was talking to a former teacher on facebook who has helped me through a lot and when asked how long it's been without cutting myself, i pretty much said i've given up on recovery and trying to stop cutting because it helps me get through each day and that is all that matters
then he said: "there does not appear to be an appropriate emoticon.....this is why I hate messaging....Grumble grumble grumble"

what exactly is "grumble"

[abraXus]

usually that person means they dont know what to say and is feeling annoyed