The plumbing pipes in your walls and ceilings are meant to be fixed securely when they were first installed and sometimes over time they can vibrate free of these fixings if not originally installed correctly. There are a few options. Firstly check to see if you can find any areas where the noise is coming from that have exposed pipes. Then check to see if the pipe clips are holding the pipework tight. If there is movement you could try to re secure the clips. Most plumbing pipes are located in the ceiling space so keep your safety in mind if getting in the roof. Another option is to have a water hammer arrestor device installed. This device allows a cushion of air into the system and can greatly reduce the noise. This must be installed by a licensed plumber. Call Lamb and Bell Plumbers for a free quote.