water noise in plumbing

Hi, everything is shut off but we still hear running water noise. The toilets, sink, washer, are turned off at their turnoff valves but the noise still is there. It only goes away when the main is turned off. I am at a loss. I tried to isolate it but could not.

Did you check all outside faucets?
Irrigation system?

Tell us some more about the house -- on a slab, multistory, basement or not, etc. Is the sound more pronounced in any specific area of the house (e.g., near where the supply pipe comes in, etc.). If you leave the main on, but turn off the hot water at the water heater, does the sound stop?

Worst case would probably be a major leak under a slab. Many plumbing firms have very fancy leak detection equipment that can find such leaks with amazing accuracy. One such I can recommend in Florida is Sleuth (http://www.sleuthplumbingtechnologies.com/leakdetection.htm) in the Tampa area.

If there's an underground leak, you might see some evidence of it, as in soft or wet ground when there has been no rain. Even if it's under a slab or in the wall somewhere, eventually it might make itself apparent. Your water bill should be significantly higher, and if it's a hot water leak, your gas bill should be much higher as well.

Try to isolate the noise a little better by taking Mikey's suggestions.