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I can see/hear water running through the pipe just under the basement floor drain in the laundry, furnace and hot water heater room. Nothing is turned on in the house. If I turn off the main water inlet to the house, the water in the above mentioned pipe stops after a minute or so. No leaks in toilets or faucets. What could be causing this?

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    Does your water meter move when you aren't (knowingly) consuming any water? May 22 '16 at 22:30
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    Well, it's not magic... I'd start closing shut-off valves for toilets, washer, dishwasher, etc., all appliances connected to the drains, one by one, and check if the water stops. Sometimes water leaking into toilet is not evident, especially if the flow rate is really slow. If your shut-off valves are old, though, be prepared that they might start leaking once you open/close them.
    – haimg
    Aug 24 '16 at 13:36
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    Any chance you have an RO system or water softener that is in back flush.. Oct 28 '16 at 22:37
  • Or a condensate line... Or in some areas a sump pump.
    – keshlam
    Oct 28 '16 at 22:45
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in some older homes, the sanitary sewer and stormwater drain lines are tied into one line out to the street mains. you could be hearing/seeing drainage water from your exterior foundation weeping tiles draining into the main drain. its often set up so that the basement floor drain acts as a vent, overflow and inspection point just upstream of where it dumps into the gas trap at the entry to the sanitary sewer portion of your household drain.

you can also look for a small plastic line that runs from somewhere just after the main shutoff down into the concrete floor. this small line is designed to constantly flood water into the floor drain to ensure there is always water in the subslab trap at the aforementioned spot. this ensures you dont get a dry trap and get methane coming up into the basement from the floor drain.

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    To clarify, the scond paragraph is refering to a trap primer. They are not designed to run constently but they often end up so as they offten get stuck open. There are diffrent types, most work on a pressure drop in the water line that allows the trap primer valve to open for a short time. They are usualy on the cold line that feeds a fixture. Depending on when the house was built, this would be my top guess.
    – pdd
    Oct 28 '16 at 22:58
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Watch your water meter. For example, check the meter before leaving the house for the workday or something and make sure that all water sources are turned off and nobody uses water for anything, then when you get back home check the meter again.

If the number is different, then you probably have a water leak. If you know that all water sources are turned off, then the meter should not have changed at all.

If the number is the same, but you still see/hear water in the drain, then maybe it's one of the things pointed out in the other answer.

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