I also had a similar problem and through my investigation I found that when the home was built, the plumber stubbed in a clean-out during construction, but did not finish it through the wall to the outside. When the brick was applied the 2" PVC clean out rough-in was covered up with plywood. This 2" open pipe has been venting in my wall for 13 years. The smell would come and go, most noticeable during humid rainy weather. I have cut the hole in the exterior wall and finished the clean-out like the way it should have been done years ago when the house was built. ($2.50 for materials from home depot). I suppose it is easy to overlook an item like this during construction. I think the rough in plumber had a good idea by putting in a clean-out by the kitchen sink but didn't follow through.

You piggybacked on a thread that was years old so I gave you your own.
You say you had a open pipe inside your wall. Was this a vent? Did you complete it and run it out the roof? You say " I finished the clean-out like the way it should have been done years ago when the house was built. ($2.50 for materials from home depot).". But failed to tell us what you have done. What about that "open pipe"? What did you do about that? Inquiring minds want to know! Cheers, Tom