re: Any plumbers out there?

Very likely a toilet. Turn off water access to them and go check the meter. It could be something as simple as changing out the flapper in the tank. If the meter is still running, check around your yard for water. It could even be a water hose... if you're not seeing water anywhere, you might have a line leaking under your foundation. If that is the case, you will need to get a leak detection done