I need help with a barking Papillon...?

My pap is 4 and a half. He was a rescue at 6 months old and came to me very fearful, timid, and shy. He has made a complete turnaround and ended up as a very fun, flirty, socialite. He's obedient, friendly, and well-mannered in public and most of the time in the house, except when someone comes to the door, walks by the window, or surprises him (accidentally, of course). He starts barking an piercing alarm bark at the first sign of a visitor, or one of us even entering the door. He will NOT stop at any command. He continues to bark even after someone enters for like 5 minutes. I have tried calm treats training, the closing of the door at a bark, even screeching and shaking a can of change. He acts possessed and transe-like while hes doing it. Once he stops, hes his sweet trotting self, but its just driving us crazy! Any suggestions would be appreciated! Thanks