Possible causes for sound of water running through house pipes when water is off?
I suspect the sound is from the drain pipes, and comes from a leaky toilet, beacause that's the most common thing that has a stored reservoir of water to keep leaking after you shut off the main valve.

You might try leaving the main water valve shut off for several hours. If the sound eventually stops, go around and look in your toilets to see if any are now empty in the top tank (without you flushing) - or speed the process up and flush all the toilets after shutting off the main water valve and see if the sound then goes away shortly after that.

If you'd like to check on @Tester101's theory, try shutting off the main electric breaker and see if the sound stops or not.