is there any chance you can video him for us to see?
I can try but it might take a while to catch him.
Best way to describe it is he drops his head so his body is basically horizontal and starts chewing really loudly (same noise as someone eating with their mouth open) and he seems to be concentrating hard on doing what hes doing as he doesn't like interruptions. If I say something he will let out an angry little squawk back to me or if i try and give him head rubs I get a nip.
Almost always he does it last thing at night before bed.
Anyway I will keep an eye out and try and film him.