How stopping water flow can sound like a gunshot in your pipes — and why it's a real structural concern, not just an annoying noise.
A washing machine finishes filling. A solenoid valve snaps shut in a few hundredths of a second. And somewhere in the wall, the pipes let out a loud, metallic bang — sometimes one thud, sometimes a rattling series of them fading out over a second or two. That's water hammer (hydraulic shock): the sound of a column of moving water being stopped almost instantly, and the energy it was carrying having to go somewhere else.