Snapping/Biting teeth during sleep?

My boyfriend, when he sleeps, he constantly snaps his teeth (as in like a crocodile) and makes noises with his mouth almost like hes sucking on the inside of his mouth. He does this all the time and Im really curious, especially the teeth snapping. Ive googled it and the only thing that comes up is 'bruxism' and 'tmj' which do not sound like it at all... I wouldnt of thought it would be any kind of disorder or health problem but I guess its possible... any ideas?