Parrot Attacks

Hello, we have a lovely, affectionate sun conure - Sunny, who is about 2 years old. He is free flying, toilet trained and very tame with the family. He loves a fuss and a cuddle and will even come under the bed covers with us! However as soon as any visitors arrive he attacks them. We cannot have him in the same room as he will bite the guests and draw blood. Even if we put him in his cage he will not stop screaming. We have tried the click technique which worked with 1 visitor but the next time they came round he seemed to forget and attacked them. He will not accept treats from guests and repeatedly tries to gnaw his way out of his cage so he can bite them.
   
History: We are Sunny's 3rd home, we have had him now for over a year and intend to keep him for the rest of his life. We sort of rescued him from a Vietnamese family who locked him in his cage all day with no interaction (not even a radio) and never cleaned him out. He has now come to a loving family and as I work part time he has company everyday.
   
I think the history of Sunny's aggression towards visitors is that we have had a few (quite loud) birthday parties, and since then he has been biting. He must have been scared at one point and will not tolerate anyone in the house. Unless he is territorial or jealous? But I do believe he has anxiety.
   
Does anyone have any advice on how we can train Sunny to stop attacking visitors? We love him dearly and want our guests to enjoy him as much as we do. We dont want to keep shutting him out on his own.
   
Please help.....
   
Thanks for your time.

It's a normal behaviour for a sun conure to be aggressive towards guests. They're unknown intruders, they do not live in the house. This has nothing to do with the history of the bird. A sunnie will learn to tolerate people or animals who live in the home. They won't tolerate people or animals who don't live in the home.
   
My sunnie is exactly the same & the only way I get around this is to have her cage in a separate bedroom so I am able to shut the door when we have guests. It stops her noise. When I have guests I put her in the bird room, in her cage, leave the cage open but shut the bedroom door. She remains quiet the whole time guests are here.