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:
http://www.youtube.com/watch?v=UODyZz2OeIU&feature=mfu_in_order&list=UL

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

Is there a PRV,(pressure reducing valve) installed where the water enters the house? How old is your house? That noise is caused by vibration can you feel where it's the strongest? Back to you. Tom PS. It didn't sound like water hammer to me either but refreshing the air chambers might cushion the noise. Would you like me to show you how to recharge your air chambers? Regards, Tom

There is a pressure reducing valve just before (or just after) the main water shut off. However the knocking sound is about 10 feet above where the valve is. As far as I can tell, by that point the main water supply has branched off to the water heater and the outside spigots, but perhaps that doesn't matter. The house is 13 years old. As to refreshing the air chambers, I thought that draining the entire house of water (main valve off, open all the faucets, flush toilets until dry) and then turning the water main back on is supposed to do the trick, but I'll take any advice I can get. The noise has been going intermittently for a few months now.

If it is a bad PRV, how do I prove this before having a plumber replace it?

Thanks

When the noise starts put your hand on the PRV and see if you can feel it vibrate. Is there anything above the PRV where you hear the noise the loudest? Let me know what you find. Somewhere the vibration will be the strongest. Feel around and let me know what you find. Good luck, Tom

Hi Tom,

So, for reasons I also don't understand, yesterday between 6pm and 10pm the knocking only happened once. Other times, it might happen on and off for over an hour.

I held the PRV while the knocking was going. The pipe feels more like it is transmitting the noise, than actually shaking. I can not visibly see the pipe move. The noise/vibration going through the pipe (the pipe bringing water into the house) was the same below, at, and above the PRV. However, as I said, the knocking seems to originate from about 10 feet about the PRV. I'll try leaving a ladder near the pipe and see the next time it knocks.

Thanks...