Try to track it down. Frequently it will be centred on a particular carpet/curtain/piece of wallpaper. Then wash it or replace it. Generally these odours are caused by moulds, which need damp conditions, so to prevent recurrence dry the room thoroughly. Drying the room will help deodorise it too. You can buy or rent a dehumidifier to do this. Sometimes the smells can be caused by dry or wet rot in the wood, in whcih case theres not much you can do....