One day, Sunny did that non-stop for 10 minutes. Doesn't sound like much, but think about it - 10 minutes of opening and closing the beak.
He could either be yawning, doing something with his crop like regurgitating, or be sick. Well I actually figred it was him trying to imitate a sound after I heard him imitate the sound that Skittles always imitates lol.
Could he be trying to talk, possibly?