If you hear rhythmic ticking sounds coming from behind the walls when someone turns on the water in your home, you may feel like you are in the middle of the story about the telltale heart. However, you'll feel better once you know the true culprit of the noises. Hearing the noises usually isn't cause for worry, as they're most likely due to an issue you can resolve simply.
One possibility for a ticking sound coming from your walls when the water runs is a problem with the water meter. If you notice the ticking sound near the point in the house where the water line enters the home, then the meter could be the culprit. If it happens anytime the water, cold or hot, is on and stops when it's not, the likelihood increases. This ticking is actually the sound of the water meter doing its job, measuring the amount of water your household uses. Most water meters are not close enough to an interior wall to make the sound come through, but it is possible in some cases.
The far more likely reason is that hot water is making your water pipes expand. You'll probably notice the ticking when someone runs hot water somewhere in the house. As the hot water travels through the pipes, particularly in PVC pipes, the pipes expand and may rub against a part of the inner structure of the house, such as wooden beams. When they cool, they'll shrink back to their original size and the noise will stop. You can take steps to stop this noise.
Keep the pipes from bumping into other parts of the house by supporting them with hanging guides or hooks and keeping them away from wooden beams or other parts of the home. If pipes are held in place in a way that prevents them from reaching the nearby surfaces, they will be unable to tap against them.
You can also insulate your pipes to keep them from rubbing against any other material. Wrap the pipes in foam or pipe insulation to stop the ticking. Even if this doesn't completely stop the noises of pipe expansion, it should muffle the noise to a barely audible or inaudible level. The only other option is to lower the temperature of your hot water heater to a level that will not expand the pipes as much.