#1 Constant running water sounds?
If no one is using the bathroom or the kitchen, hearing running water sounds is always a bad omen. It means that you can have anything from a burst pipe in the walls to problems with the irrigation system or even maybe have a busted pipe under the floor (if you have a heated floor system, it could also mean that one could be leaking somewhere too).
The first thing you should be doing is trying to pinpoint where it’s coming from. Then, once you have the place located, go ahead and close off the main water line. If the sound goes away, it means you have a leak in the pipe behind where you discovered the sound was coming from. And unless you are really handy and ready to tackle this problem by yourself (which we generally do not recommend), you should call your plumber as soon as possible and get the issue resolved before it gets worse.