Maybe you could start with something like, "Betty, you may not be aware of this, but you tend to talk to yourself a lot, and it distracts me." You said she would probably want to know and try to stop if she were made aware that it bothered someone, so that's good. But, if it's such a habit that she keeps doing it, maybe in a second conversation you could say, "Betty, you know how I mentioned you talking to yourself and how it distracts me? Well, maybe we could work out a signal I could send you when you're doing it." Then you could tap on your shared wall or send her an IM or whatever works best in your setting.