Yes he is young and in the process of weaning, I offer formula to him once a day, and he has been eating millet and attempting to eat his pellets.Welcome to the forums
Is he still young? I don't know conures well - hopefully someone pops on soon that does.
I did find this article online about sun conures vibrating their beaks as a sign of affection (I didn't find other articles with the same thing you're describing so I don't know how accurate this is)
Sun conure mouth vibration