knocking sound only when water is off
 

As shown in the videos, the noise is a rapid, loud, rhythmic beating that can change in tempo, duration (usually 30 seconds or less), and volume. The noise always comes from the same location in the house - it seems to be in the interstitial space between the basement and first floor, roughly near/above (but not in) the water heater, and this happens to be near the spigot at the back of the house.

Before the noise starts, no water is running. None of our toilets are leaking, no faucets are dripping, no sinks are running etc ... In addition it does not correlate with the automatic ice maker so far as I can tell.

Having said that, once the noise does start, if any water valve is opened (faucet, back spigot, toilet etc) the noise stops instantly. Some times it comes back in a few minutes, some times it doesn't start again for hours. Usually we hear it at least once a day, but that depends on how much we are home.

Things I have tried include flushing the water heater (although I don't think it is minerals in the water heater) and draining all of the water from the house in case this was a water hammer issue (both plumbers didn't think it was this). I've tried each several times, but neither has helped alleviate the problem.

The most recent thing we have tried came at the suggestion of one plumber. On his advice we asked the county to give us a new water meeter. They claim to have done this, but the noise is still there.

Here are two videos of the noise:

So I'm hoping someone might have an idea or two about what is causing this noise. I can call another plumber but paying some money to have someone come out and say "I don't know" doesn't seem productive.

Thank you for the help

knocking sound only when water is off
 

Do you have any check valves in you cold water lines. Any check valves between the cold inlet of water heater and the well tank? Does your water heater relief valve ever pop off? Paul

Bet you have a leaking flapper in a toilet...
And a korky fill valve....

Re: knocking sound only when water is off
 

paul52446m, there is a PRV valve where the cold water comes into the house. Can this be the cause of the noise? If so, how do I prove it and can I easily replace the valve?

Redwood, If it were a toilet, I should be able to sequentially turn off the water to all of the the toilets and eventually find if that is the cause correct? I'll give that a try, but I've been listening pretty closely and never heard the water running to refill a toilet.

Thanks
Avram

paul52446m, there is a PRV where the cold water enters the house. If this is the source of the knocking, how do I prove it and how do I replace it?

redwood, if it is a leaky toilet, then I should be able to isolate the problem by sequentially turning the water off at different toilets. I've listened pretty closely though and I don't think I have a leaky toilet. The water bill hasn't changed much either ...

Thanks
Avram

Have you looked at the water meter while the knocking is occurring?
I guarantee that it is spinning...

I don't have an answer so I will ask a question. How much expansion can you get from copper pipe when it heats up and if it were in a tight hole could you get noise when it heats or cools.

knocking sound only when water is off
 

Your idea is the two things i was looking at. One if the water heater comes on so the hot water has to expand some where. I had one where they put in a softener system and they had a check valve on the cold line so the water heater could not expand back into a expansion tank. so it would pop off and bang the pipes. I put a expansion tank on the hot side of the water heater.
You are right on a hot water copper line going through a tight wood hole. When it heats up it can make a real loud noise as it moves in the hole. Paul

Have you tried shutting off the water completely to absolutely confirm it's the water lines?

We had an almost identical problem here at a home in my town. The noise was usually at about the same time in the morning, right after they woke up. Happened before use, seemed to quiet after water was turned on. We changed out the meter with no luck, then removed the meter completely. Still not a fix.

Suggested to the fellow to try changing out his PRV and left it at that.

Spoke to him about 2 weeks later and asked him how he made out. Turns out it was a woodpecker banging on the back of the house :D

I wish it were as a simple as a woodpecker. But the knocking is as likely to occur at 10pm as 10am.

I have had the water off to the house for periods of 8 hours and not heard the knocking. When I do hear the knocking, the water to the house is on, nothing is running or leaking so far as I know, and it is independent of the on/off status of the AC and heat (I thought it might be something lose in the HVAC system).