Strange pounding

This time the unexplainable occurred in my wife's store which is located in a very small village in Ohio. We have had several unexplainable episodes in this old 1800's structure. This time it was pounding noises, not unlike someone pounding on the outside wall with their fists. The episodes happened several times today. We even (one of us) stayed outside to see if we could figure it out. Even with one of us in plain view outside the pounding continued. No one but us were in the store for most of the episodes but they occurred twice with customers present. One customer commented that we must have someone working next door. There is a shop next door that repairs automobiles but it is for sale and is only open sporatically. Today it was not open and besides the pounding did not sound like a hammer but more muted like a fist. Besides that the pounding sounded like it originated on the opposite side of the store from the closed shop next door. The pounding ended in the early afternoon. still unexplained. If the pounding was outside, like against the wall of our store, the person must have been invisible. The sounds erupted even when one of us was outside to witness it. The sounds seemed to be coming from the parking lot side of the store. There are no structures on that side of the store for quite some distance. If the sounds were originating from somewhere inside of the store; where? No, there are no water pipes on that wall from the side entrance to the store front. In fact, the store was built lathe over studs, a simple shell on that side. There was no wind today which is extremely rare for this area. So when the wind isn't blowing we really notice the fact. I suppose there is an ordinary explanation for the pounding; but we certainly couldn't find it.