Why is my rabbit constantly sneezing without other cold symptoms?

Hello. Rabbits very commonly have upper respiratory infections. They can be from dental disease, specifically the roots, or sometimes there is a primary rhinitis. Rabbits make a very thick pus that doesn’t drain well, so when an infection is present inside the nose or nasal passages, it can be extremely hard to clear. Sometimes surgery or anesthetized nasal flushing is required. Since nothing has really helped before, I would recommend a thorough exam by a veterinarian along with blood work, skull radiographs, a deep nasal swab or nasal flush for cytology and culture. From there, any additional procedures can be recommended (dental trim, extraction, rhinoscopy, etc). Likely, Hangy will need a long course of medication (4-8 weeks) and may have relapses throughout his life. At home you can make things a bit easier on him by reducing airway irritants, keeping his cage clean, and supporting good dental health with high quality grass hay. Good luck!