It's seldom that pipes feeding a toilet come out of the wall, they are usually drilled through from the basement. The drain certainly can't be going into the wall, that's just not done. So, if there is actually a sound coming from inside the wall behind the toilet when someone is on it, then there must be pipes in there that go somewhere else. Could it be that the only time you or anyone else is close enough to that wall is when you're on or near the toilet? Otherwise, there would have to be a fitting in a line in the wall, that had come loose to the point that it separates just enough from the flex in the floor, (of which there shouldn't be any), and leaks the drips that you're hearing.
If it were the flapper ball in the toilet tank, the water wouldn't make a noise when it goes into the bowl, it just runs down the side of the water channel and into the rim where it dribbles out from the holes under the rim. You'd also be able to see it when you look into the bowl as you lift the lid. The water in the bowl should be like a sheet of glass, (no ripples).