My husband and I adopted a six year old Scarlet Maccaw that was handfed by its previous owner, who had become too ill to care for her anymore. When we brought her to the house, we set the cage up, exactly as it was before in a quiet spot and spent a lot of time with the cage door open, sitting far away, but within her view, just to get her curious. Each day, we would move a little closer, with the door still open, until she finally poked her head out and gathered up enough courage to explore her new environment. After that, we slowly added new toys, moved her to a more active part of the house, offered her treats, and now she is as noisy and content as ever and she has learned how to vocally outperfom just about anyone.
Since a Sun Conure is smaller, it might help to put the cage up high, so the bird will feel more at ease. I don't know if any of this helps, but patience and starting out with a calm yet social environment can work wonders. Do you know if the bird is a male or a female? Also, keep an eye out for neurotic behavior like plucking. Hope everything works out! I'll dig up some of my books to see if there's anything else in there.
