This is a very peculiar situation, although JRTs do tend to have their quirks. There's two possibilities that I can think of here. Either there is in fact something in the wall, perhaps a dead rat, in which case you'll either need to get it out or wait for it to decay on its own to solve the problem.
Another possibility is that there was something in the wall at one stage that's now gone, and your dog has become hung up/obsessed with it and thinks it's still there. In this case, training is the answer. You can try training the behavior out by distracting her attention away from the wall with games and basic obedience training exercises. Every time she indicates that she's trying to get at something in the wall, immediately break her concentration with a clap of the hands and move into a game or some quick training.
Another option is to set up an imaginary "out of bounds" area around the wall. I explain how to do this in my ebook.
