Can't figure out knocking pipes...

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Soulbarn

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so, we've got knocking pipes—always from what seems to be (since they're in the wall, can't be absolutely certain) the same general vicinity—the pipes that lead vertically from our downstairs bathrom (tub, sink, toilet) to our upstairs (sink, toilet). The knocks are sometimes quite loud, sometimes soft, usually four to six in rapid succession. They're not totally metallic in tone—our kids always think there's somebody at the door.

I've tried a bunch of troubleshooting stuff, like:
- draining the whole system by turning off the water into the house, opening all the faucets, low to high.
- turning off the different items in the two bathrooms, systematically (one night with the downstairs toilet off, then the next night with the sink, etc. in various combinations).

Those don't seem to have done anything; the sound appears to be intermittent and not related to any particular appliance or attached item being on or off, either concurrently or recently.

We did have quite a bit of work done over the summer—new kitchen with new hookups for sink/dishwasher/washer/dryer; plus a water softener system. The sounds started a few months after the work was done. I've tried bypassing the water softener, as well, with no noticeable change.

Any other thoughts? Am I risking some kind of internal pipe damage? Any advice on what I should be looking for when/if I call a plumber?

Thanks!
 

Reach4

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Those don't seem to have done anything; the sound appears to be intermittent and not related to any particular appliance or attached item being on or off, either concurrently or recently.
Do you have a submersible well pump? Many people, including me, cannot hear when the pump goes on or off. You can tell by looking at the gauge going up or not. If close to the switch, you can also hear the pressure switch click on and off. Maybe you can correlate the sound to that.

I suspect you have a loose pipe vibrating due to something.
 

Cacher_Chick

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More often than not the cause is a pipe or pipes in the wall that were not properly secured to the framing members. When water is turned on or off, be it from a toilet, sink, washer, or other component, piping that has the ability to move will move, creating noise. When the piping is banging against a framing member, the sound can be transmitted throughout the adjoining framing, making the noise seem even worse than one might expect.
 

FullySprinklered

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Most often, the tapping noise comes from the water lines expanding when the hot water runs through them, rubbing against the j-hooks or clamps that hold the water pipe against the joists and studs.

One other possibility comes to mind, and that is that the pressure regulating valve is going bad. It's a rare symptom, and I've only seen it once. The PRV was hidden in the wall near the washer. Dealing with pressure issues comprises maybe 15-20% of my plumbing service calls. I haul maybe four homer buckets of those darlings to recycling in a year's time. Thanks for the work, Mr Watts.

Next time you feel like fronking with it, turn on the water and go down and put your hand on the PRV and see if it feels like the source of the noise. If so, replace it.
 

FullySprinklered

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More often than not the cause is a pipe or pipes in the wall that were not properly secured to the framing members. When water is turned on or off, be it from a toilet, sink, washer, or other component, piping that has the ability to move will move, creating noise. When the piping is banging against a framing member, the sound can be transmitted throughout the adjoining framing, making the noise seem even worse than one might expect.
great minds think alike
 

Dj2

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It's most likely the reason stated above. Correcting it may be hard, since you need access to the pipes in order to fasten them properly.
 

Soulbarn

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If it is loose pipes in the wall, and I don't fasten them, am I risking damage? (I may be able to access them—there are a couple of panels in the closets, but I've never looked behind them, since they're well-fastened...)
 

Reach4

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Try correlating the sound to toilet flushing and bathroom water use. Does the sound ever occur when nobody has been in the bathroom for the last few minutes?
 

Soulbarn

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Try correlating the sound to toilet flushing and bathroom water use. Does the sound ever occur when nobody has been in the bathroom for the last few minutes?

There seems to be no correlation between any water usage—that's after many nights of trial-and-error...
 

CountryBumkin

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If you can locate the place in the wall where the knock is coming from you may be able to use some expanding foam to secure the loose pipes.

Perhaps you can poke a little hole in the drywall and insert the straw from the foam can. I don't know if the foam will fill-up/expand enough to hold the pipes or if it will fall to the bottom of the wall cavity. Might be worth a try - might just make a big mess in the wall.
I'm not a plumber.
 

Reach4

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Do you have a water pressure gauge? Consider getting one with a lazy /tattletale hand that records the peak pressure. I say this because fullysprinklered suspects a PRV or thermal expansion. You can find those locally with a garden hose thread for under $20 usually. That can attach to a hose spigot, a laundry tap, or even the drain on the water heater.
http://www.homedepot.com/p/3-4-in-Plastic-Water-Pressure-Test-Gauge-DP-IWTG/100175467 is an example.
 

Soulbarn

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Well, I got into the panels and found the pipe—I was able to replicate the sound by jiggling it. The cause of the sound is unclear, but I did find rodent dropping around, and the pipe descends beneath the house; that the sound seems to be more frequent at night (I thought this might be my imagination, ascribing it to the fact that I'm around less during the day, but now maybe I think my impression was correct) may also possibly indicate a rodent culprit—maybe the critters are climbing up to the attic from there. Or maybe it isn't rodents; but certainly sealing off the area will help to prove that one way or the other.

So my plan is to close off access to the crawl space with some steel wool. Then I have to figure out a way to secure the pipes. I've attached a picture. Though I can replicate the sound, whatever the pipe is knocking into is far above the access panel, and out of reach. Any thoughts on how best to secure it? In the image below, the pipe that seems to be causing the problem is the one on the left.

IMG_6359.jpg
 

Reach4

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may also possibly indicate a rodent culprit—maybe the critters are climbing up to the attic from there. Or maybe it isn't rodents; but certainly sealing off the area will help to prove that one way or the other.
I have a friend who had rats under the barn and chicken coop. Common baits did not do the job. Traps were not keeping up. He went to a professional exterminator supply place and bought some expensive stuff. Throw out the bags. No need to open; they open the bags themselves. Results were very quick.

I don't know the name of that effective stuff.
 
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