We can hear water running in our pipes, pretty significantly. The meter shows the water usage at a pretty fast clip, even when no water is on in the house. The city came out and checked their side of the meter as well as in and around it. Previously, I would hear it but it would usually stop. Then it ran for 3 days straight without stopping. That's when we called the city to check, but they couldn't find anything and just turned the water off to the house. Estimate of water lost up to the point was about 10,000 gallons.
I called a professional plumbing service and they came out after a few hours. He checks around first for obvious signs of water and finding none, turns the water back on. The meter is no longer moving and showing water usage. I asked him to do a couple of other things since he was out anyway. He left to go get parts and when he got back the running water sound in the pipes was back. He checked the toilets, the hot water heater, taps, etc. and for any obvious leaks around the house or seeping up from the slab, and nothing. He turns the water off again so he can do a rebuild on my hot water heater. When he turns the water back on, the running water sound has stopped again.
Now we're back to it will run for a while, but then stop. The plumber really won't be able to do anything unless we can hear the running water, so it's hard to know when to call him out again, although he couldn't find it before when it was going. We even tried turning the cutoffs to the water supply for the toilets, but it doesn't make any difference when the water sound is going. Very puzzled and anxious to hear if anyone out there has any ideas.
Thanks,
Mace

It sounds like a neighbor is tied into your meter and you are paying the bill. Have you noticed a larger bill lately?

Is this a single family home or an apartment/condo unit in a multiplex?

I wouldn't rely just on the sound you are hearing but watch the meter outside instead. When the meter moves without water consumption, your plumber should at the minimum be able to determine if there is a leak or unintentional consumption, whether it is:

(1) between the meter curb stop to the house shutoff, or
(2) from the house shutoff to the inside.
(3) if it's inside the house, whether it's on the cold side or hot side of the supply.

Once that's determined, there are additional steps to further narrow down where the consumption is happening.

In addition, disconnect all garden hoses from outside bibbs, turn off all timered sprinkler valves.

Hi, Thanks for your responses. According to the water meter in the yard, yes we're using water even when there is no consumption in house. Turning off the water at the meter stops the sound. In the past I've heard the sound before, but it always stopped. This time it stayed running for 3 days straight but is currently stopped. When the city came out to check they said the meter showed we were using 1 1/2 gallons per second. Everything else is exactly as I described in the original post.
We are a single family dwelling, and yes the water usage appears to be in the house, since turning it off at the meter stops the meter from moving. But we still don't know where it's going or why it starts/stops. The plumber did hear it for a while, but then it stopped again.
I don't think a neighbor is stealing water, although I did walk the perimeter of my back yard just to see if I could see or hear anything out of the ordinary. No new swimming pools put in at any of my neighbors as far as I can see.

This is indeed a mystery.

The city says it's consuming 1.5 gallons per SECOND? That is a very high flow rate. I don't think if I turn on my garden hose at full blast for 1 second it will fill a one gallon bottle. Are they sure it's not gallon per MINUTE? If you have that kind of loss 3 days straight, you would have consumed 388,800 gallons for those three days. Add to that the water loss seems to start and stop at random, makes it more confusing.

Starting at the meter, where you have a stop. If you turn off water there, the sound stops. Now, there is a pipe that runs from the meter by the street, to the house. Usually when it gets to the house, especially in the southern states where freezing is not an issue, it will turn up where you will see a riser, where you will see another shutoff valve, before it runs inside the house, there is most likely a hose bib there. If you can locate that shutoff valve, it will help to know if turning that off, whether it will stop the strange sound. If it does, then there is a leak/loss between the street meter and that house shutoff valve. If it does not, then the leak is downstream of that valve.

Another thing to check is your hot water heater. Typically at the water heater cold water inlet side there is another valve. That valve when closed will stop cold water going inside the WH, and will in turn stop the hot water side. If by turning that valve off and the sound stops, then your leak/loss is on the hot water side. Otherwise it's on the cold water side.

There is also a possibility the consumption is outside the house, say the piping runs through the house then eventually runs to a back yard hose bib and there is some sort of a sprinkler controller that comes on and off on a timer, and feeds some buried soaker hose or drip irrigation system. So when it's on you may hear water running through the pipes but you do not see actual water consumption.

In addition, that sound may or may not be 100% related to the meter spinning. You may hear water "running" in a hot water recirculating system when no water is being actually consumed.

ditto

excellent explanation

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

Do you have a swimming pool? Some pools are equipped with auto-fill hardware that automatically adds water to keep the level at the right height for the skimmers to work properly. Especially in hot Southern states, you lose a lot of water from evaporation. When we had our pool in NY, we could lose 1" per week in August, which on an 18x36 pool could be an hour or two to get from a garden hose.

Either that or some kind of sprinklers or irrigation. If the meter is spinning you are definitely using water, so the sound isn't a phantom noise. If nothing is on inside the house, and there is no water on the floor, something outside the home must be using the water.

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Not a plumber by trade but a fierce DIYer