It can't 'rain' inside as there is no weather for interior cells.
I think it's a camera effect. Is there a giant red box around the entire interior area? It would be an FXCameraAttachTrigger. If so, delete that. It attaches a 'dripping' effect to the camera that can be used to add to the atmosphere of very moist places. (Like sewers)
Or you can change the effect it uses. There are a lot of them. Dripping water, fog, dust blowing, slow blowing, snow falling, leaves fluttering about. A lot.

(Change the properties in the scripts tab of the trigger's object reference window)