Dog barks at wall

My younger, very black, schnauzer has been barking at a prescribed place on the wall that seperates the Diningroom from the Library.  She has done this several times a day for the past three or four months. It makes no sense to me and my wife is getting very irritated with her. The area of the wall she is so interested in is an area about two foot square from the baseboard up just beside the doorway that leads from the Dinningroom into the Library. The barking is intense, sometimes almost hysterical, for several minutes at a time. I have inspected the area myself, even going to the basement and checking out the wall that supports the Library/dinningroom wall. I have found absolutely no sign of anything out of the ordinary. I even tried leaving food scraps out in that area of the basement and it was not touched. I set up two of those rodent high pitched screamers and aimed both at that area of the wall, but so far, they have had no effect if it really is some sort of critter that has her attention. Strangely my elder grey and white schnauzer has no interest in barking at this wall. It has gotten so, when the younger dog starts her bark attack the older dog  leaves the room and goes into the kitchen as far as he can away from the racket. If anyone out there has ever had a similar situation let me know what it was and how you took care of it, please.