House built in 1988. Two stories. Whenever the shower or the sink in the upstairs bathroom is in use there is a popping noise in the exterior wall on the first floor (wall beneath shower/sink). It is rhythmic as well, meaning a loud pop every 1/4 to 1/2 second and continues as long as water upstairs is on. The popping sound is loud. I have narrowed it down to be in the wall but do not see any moisture on the wall or ceiling. I wanted to check here before I called a plumber. Any ideas?

Have you ever had your water pressure in the house tested? Also, do you ever notice this when the washer is filling, or the dishwasher if you have one? I've seen situations like this because the water presssure is too high in a house. Also, the wrap around brackets on the pipe could be loose where its connected to the wall stud causing vibration. You can pick up a water pressure tester fairly cheap at a hardware store or home improvement center. The average water pressure in a home should be around ~ 40-45psi... if you are higher in that you run a risk of the plumbing leaking (bursting is more like it) at the weakest points (usually water supply lines at your toilet or sinks). Just a thought, but Id start at testing the pressure.

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Yes, it sounds like Water Hammer. Try to increase/decrease water pressure. You can install water hammer arrestors under the sink. See if that helps.