Hmm, just a thought but maybe recording the noise with your smartphone isn't that foolproof. I know phones have a good frequency range, but it's also dependent on the model and software. Just because your phone can't capture it, doesn't mean it isn't there. Tried to record a "whining" wifi box once that drove me nuts, but my phone didn't pick it up. I'm in my 30s and I can hear high pitched sounds from electronics and other devices literally nobody else can hear.

Maybe try to locate the sound with better equipment....before tearing down the wall.