Water, as it is a liquid, does not squeeze like a gas; it is a non-compressible. Because of that, water acts like a solid in transmitting sound. When waves are started in one place, the pressures of those waves are felt everywhere. To start a vibrating pipe, all that is necessary is to have an unsupported piece of the right harmonic length. When the tap is opened to that certain spot, the water passing over the valve body is similar to air passing over an organ pipe reed, the vibration is carried throughout the pipe.
So find where the pipe is unsupported and fix it down with pipe straps.