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  1. #1
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    Sep 2009
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    Default Knocking Drain Pipe

    When i use either the sink or bathtub in our main bathroom, as it's draining there is a knocking in the drain pipe that continues as long as 5 minutes after we are done running water. It happens worse when you use straight hot water and it's worse on the sink. When i put my hand on the pipe in the basement where it comes down through the floor i can feel it but the pipe really doesn't move much. I was thinking maybe a venting issue but everything drains great and never bubbles, gurgles, or surges as it's draining. There is another bathroom on the same drain line but it does not cause the issue. Started happening about 2 months ago. I'm stumped!

  2. #2
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    Default Re: Knocking Drain Pipe

    What you are hearing is the pipe expanding from the hot water and at some point it is passing through a tight hole in a stud or floor joist. To solve this you must first locate the point of the restriction and free it up. If possible.

    John

  3. #3
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    Sep 2009
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    37

    Default Re: Knocking Drain Pipe

    Quote Originally Posted by johnjh2o View Post
    What you are hearing is the pipe expanding from the hot water and at some point it is passing through a tight hole in a stud or floor joist. To solve this you must first locate the point of the restriction and free it up. If possible.

    John



    I understand what you are saying, however it happens even when it's cool or cold water and nothing has changed between when it didn't happen and when it started happening.

  4. #4
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    Default Re: Knocking Drain Pipe

    It doesn't matter if the pipe expands from heat or contracts from cold, if it causes movement in the pipe it will produce the noise.

    John

  5. #5
    Join Date
    Jun 2011
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    208

    Default Re: Knocking Drain Pipe

    Scfd529:

    You're saying the DRAIN pipe is causing the noise, but Johnjh2o is saying that it's the water SUPPLY pipes heating up or cooling down that are rubbing against a stud or something that's causing the noise. Do some testing to find out if it's the supply pipe(s) or the drain pipe. Fill the sink with hot water and DON'T drain that water, and see if the noise still occurs without any water going down the drain pipe. Similarily, fill a pail of water in your laundry room sink (or any other source of water OTHER than your bathroom sink or tub) and pour it down your bathroom sink to see if that strange knocking re-occurs. By doing a few tests like this, you can nail down whether the noise is associated with the supply pipes or the drain pipe and whether it's a temperature related issue.

    To check (and possibly find the problem) go to any store that sells automotive tools and ask for their least expensive "mechanic's stethoscope":



    You should be able to buy one of these for $10 to $15. Mechanics, and pretty well everyone who works on machinery of any type uses these to find the source of noises. When you touch the probe of the stethoscope to the part of the machine that's making the noise, that same noise will be heard loudest through the stethoscope earphones.

    Touch the probe of the stethoscope to your hot and cold water supply pipes (as well as to the drain pipe) while you're hearing that noise, and that will tell you which pipe (or pipes) are rubbing. Also, if you use a stud finder to locate the studs in your bathroom walls, you should be able to touch the probe of the stethoscope to the wall where each stud is and determine which stud the pipe is rubbing on.

    Now, if it turns out that the noise is coming from the hot water supply pipe, it's most likely expansion and contraction of the pipe. Post again if you determine that the noise is actually coming from the drain pipe since one wouldn't expect much change in temperature in the drain pipe from sink or bath water draining through it.
    Last edited by Nestor; 07-03-2011 at 06:43 PM.

  6. #6
    Join Date
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    Default Re: Knocking Drain Pipe

    Five minutes is quite a long duration where 30 seconds upward to a minute is more common.

    I agree with johnjh20 regarding the expansion and contraction of the drain line rubbing being the common problem. Typically it is more of a clicking ( ticking ) sound that's heard.

    However, a knocking sound points to a loose pipe that is not supported properly. When large volumes of water drain, the pipe will move and bang against the studs within the wall or even the wall itself if there is any space or against joists if run under the floor.
    It may be at the point where the sink drain pipe goes through the wall or the floor and may connect to the 2 inch line from the tub. Or the 2 inch tub drain pipe itself is loose or bellied. Check the drain line from the tail piece on the sink to see if it's moving down when you drain the tub.

    Another thing you may find --- supply pipes and drain pipes that run right next to each other are striking one another and creating a clatter.

    2 cents worth.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  7. #7
    Join Date
    Sep 2009
    Posts
    37

    Default Re: Knocking Drain Pipe

    thanks everybody for the responses. It is the drain pipe for sure. When i go to the basement (have a single story ranch) and put my hand on the drain pipe when it's happening i can actually feel it. It comes down through the wall from the sink and 90's where it turns into the 2 inch main and is supported shortly after that 90. Just down the line it picks up the toilet and bathtub from that bathroom and then goes over to a bigger main where it comes together with our 2nd bathroom which is just a half bath and then goes down under the basement floor and ties into the rest of the system. I will post some pictures and see what i can find out about where it comes through the floor. It looks to me to be supported well enough that it shouldn't have settled over time to start causing the issue all of a sudden. Keep an eye out for an update.

  8. #8
    Join Date
    Aug 2007
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    Default Re: Knocking Drain Pipe

    Good -- you have access from below.

    You should also check the drain pipe connection at the tub drain tail piece as well.
    Can you pull down on the exposed drain pipes to see if they move ?

    Just thought of another scenerio -- could be the drain pipe is moving and may be pulling on the horizontal vent line that may be present in the wall above the sink .
    Last edited by canuk; 07-03-2011 at 10:33 PM. Reason: correcting my poor spelling
    "" an ounce of perception -- a pound of obscure "
    - Rush

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