What is causing the rattling in the bathroom with the shower?

Answer 1

Water hammer is usually recognized by a banging or thumping in water lines. The noise occurs when the flow of moving water is instantaneously stopped by a closing valve. This sudden stop results in a pressure spike behind the valve which acts like a tiny explosion inside the pipe. This pressure spike will reverberate throughout the Plumbing system, rattling and shaking pipes, until it is absorbed. Normally, a sufficient pocket of air will absorb such a pressure spike, but if no pocket of air is present, expensive fixtures and appliances within the plumbing system will be damaged as they are left to absorb this pressure spike.

Answer 2

It's called water hammer. Water hammer is a pressure surge or wave resulting when a fluid in motion is forced to stop or change direction suddenly. Water hammer commonly occurs when a valve is closed suddenly at an end of a pipeline system, and a pressure wave propagates in the pipe.

You can prevent it by having water hammer arrestors installed in your water supply system, or just by simply turning the suspect valve off slowly, instead of closing it quickly.