Water hammer or something else?
I feel like an idiot but it turned out to be the subwoofer in the living room that apparently died while I was working on that dimmer. This is after checking the attic today, checking out the alarm system's control unit and lowering the water pressure last week. Since the sound seemed to come from the wall I just decided to unplug everything one by one and that's how I found out it's the subwoofer. Since it's against the wall the sound travels up and is amplified by the wall.

Yes I'm an idiot, but I'm glad it's solved at least. thanks for all the help and suggestions.