It sounds like you have a leak under the slab. First check the toilets to be sure there not running. If you have any lines that extend from the house (pool ,shed, outside faucets) turn them off. Then turn off your main water supply, don,t use any water in the house. After 5 min. turn the main back on. If you hear water refilling the system then you have a leak under the slab. In my area there is a company called American Leak Detection that we use to locate leaks under slabs. They locate it, expose it and put a temporary patch on the line. I think they are a franchised company, they may also be in your area. Around here the leaks are caused from pinholes in older copper tubing.