I agree with boni that going to the chiropractor is a good idea, but since there is no pain, the problem is likely localized repetitive spasms of a portion of your diaphragm.
I actually had this same experience years ago. For me, it was painful and I really only felt it when I laid down, although that spot was uncomfortable and sore when I was up and moving around. I went to the chiropractor and got my ribs adjusted and took care of the problem. Good luck!
You don't say if you have pain and it is a stabbing pain. You make it sound like we should think a ghost is poking you. What's up?