If you hear running water, there could be a pipe break between the house and the main (meter). Try turning off water at the meter and see if that makes the sound go away (it should). If you hear the sound after you turn off the main house supply valve, my GUESS is that your problem is between the meter and the house shut-off valve.
If it turns out you need to replace or repair your line from the meter to the house, get the piggy bank and take out a loan from the kids piggy too. You will need to cut and repair any concrete or paving the line runs under. Depending on your skill and complexity of the situation, it could also be DIY for the cost of trenching, pipes and fittings. You might also ask your water supply utility to inspect. If water is running on your side of the meter, you may be paying for it.