Originally Posted By: Dignan
we have a valve in the wall for our shower. It's an electronic one that's controlled by a digital panel. I've never fully eliminated this valve as a suspect, but I know exactly where it is in the wall, and if I put my ear directly next to it the sound isn't any louder.

The only (ONLY!) way to know for sure, is to turn off the breaker that provides power to that valve, and then see if you can still hear the noise.