Hello Velvet. I will do anything I can to help you but I need to get a bit more information first. For right now, stop working with him and just leave him alone, let him stay quiet so you do not add to his stress.
Are you absolutely positive this sound is coming from his beak or jaw and is not associated with his breathing?
Is he holding his beak open when he breathes?
Is his tail bobbing up and down as he breathes in and out?
Have you seen any change in the appearance of his droppings; color, consistency or amount?
Is he doing any kind of vocalizations at all?
Is he keeping his feathers fluffed up?
Please tell me everything in his usual diet.
The more information I have, the better I can help.