My female budgie always does that, its looks like your budgie is almost panting right? if thats what you mean, mine does that too... i was always worried at first that she may have some respiratory problem or something....but i think shes just afraid or hot or tired sometimes when she does that....so far shes been behaving normal other than the fact that she always breathes heavily like you describe....
Has your budgie been doing this for a long time or just recently??
I think he has been doing for some time now - the problem is I _really_ noticed it only now. That's why I got worried.
Other than that he doesn't really show any problems, he signs, eats, and chews my hand,... normally.
Does your budgie make a grinding noise when he does it? As was mentioned, this often means a sign of contentment. Or does it look more like he's chewing (but without the food in his beak)? This can be very normal too.
It is hard to describe what a budgie is doing with his beak unless you are looking at him. However, having said that ,your budgie should not appear to be breathing hard or panting. If that is truly what it looks like to you and their little chest is heaving or breathing is loud or labored you should perhaps take a video of it when it happens and have your vet take a look at it. If ,however,they open their mouth wide and stretch their necks a few times that appears to be normal. If they also move their beaks as if they are grinding their teeth and you hear little grinding sounds, that is normal contentment sounds. They do this just before napping a lot. They are happy when they do this.