Chewing behaviour

My 9 year old dog does this strange behaviour nearly as if he's chewing but not quite, the opens and closes his mouth very quickly and makes a huffy noise, it's kind of difficult to explain. He only does it sometimes and I haven't noticed any triggers, or anything that causes it.

Hello and thankyou for this interesting question.   I think that it would help to see this behaviour; perhaps you could try to capture it on video for your vet.  Things that I thought of however, include dental disease, sinus disease, oral pain ir temperomandibar pain or even fitting / neurological symptoms ( which can certainly occur in the absence of other signs). Therefore, this is a sign that ought to be taken seriously, even if it appears to be trivial or short lived.  Please do talk to your vet.