I am by no means an expert but I read another thread about this. Many believe it can be the sound of an angry hamster or a content hamster, depending on the situation.
You've had him a while and can hold him so I imagine it might be content but with the bat chewing maybe she is annoyed whilst in the cage? It depends why the bar chewing is happening I suppose.
Others will give a much better reply. I'll see if I can find the other thread for you.
EDIT: here's the link
Teeth Chattering
Sorry I don't know how to make it clickable if it's not already! Most people seem to be going with the irritated theory but there are some other examples too. Hope it helps.