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Strange knocking inside wall

tor123 | Posted in General Questions on

I recently had some insulation and drywall put in my basement- and lately I’ve been getting a knocking sound (usually in the morning but not always) that last for about 5 minutes. When it happens early in the morning (5am) – it’s enough to wake us up. Any idea what the sound might be? I attached a clip.

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  1. charlie_sullivan | | #1

    I couldn't get your clip to play but I wonder what kind of heat you have. Could 5 AM correspond to when a thermostat setback ends and pipe start warming up?

    1. tor123 | | #3

      I have HVAC heating. I have my thermostat set to a constant temperature it’s such a strange sound. I don’t think it’s dripping but might be expansion/contraction. It’s been pretty cold in Toronto (Canada) the last week or so.

  2. Expert Member
    Michael Maines | | #2

    The sound is faint and does not sound exactly like knocking heat pipes to me, but that's the most logical explanation I can think of as well. Another cause of knocking is "water hammer" when using plumbing fixtures but that is faster. Some boring insects (hole-makers, not dull) make an audible tapping sound on a slow frequency like you have, but not very loud.

  3. maine_tyler | | #4

    Coming from ductwork then? What's that humming sound in the background.

    You can use a stethoscope to pinpoint source location.
    New framing lumber in basement?

  4. onslow | | #5


    The tag on the file seems to be amiss. I got it to download as an MP3 and could hear it after turning up my internal speaker levels. The sound is very, very similar to the sounds I got from my new PVC drain lines when a hot bath got drained down. The cure was slipping a piece of HDPE tape under the pipe where it dragged on a framing member. I could not lift the pipe enough since the drag point was right next to an elbow to the stack going up the wall, so no flex left in the pipes. Didn't stop the dragging, but did dampen the sound.

    If you have a high efficiency furnace with a pvc vent pipe, I would first look to see if the path takes it across/through any framing and also check the exit point in the rim joist if it goes out at that level. The earlier morning timing might be coincident with low temps falling about that time and the furnace kicking in. The very warm vent gases would certainly cause the pipe to expand in length. Listen for noise just after the furnace kicks off. It may be that the noise is always there, but masked during the day by other activity. Things that go bump in the night are always heard loudest from bed.

    The early hour of perception rules out baths unless someone in the house works a very early shift. If that is the case, then you will need to trace the drain lines. Happy hunting, these things can be very annoying to find.

    1. kevin_in_denver | | #6

      I recently tried to fix this same problem. I cut open the drywall to take a look and made sure the hole would accomodate one of these:
      I thought that if I sprayed a lot of WD-40 on the pipe where it was rubbing on the wood, it might silence the noise. It didn't work, and I'm at kind of a loss.
      It's a super common problem (shame on you plumbers) what's an easy fix?

      1. charlie_sullivan | | #7

        If you want to lubricate PVC against wood, there are a lot of lubricants that would work better than WD-40 which is so thin it mostly evaporates; the rest soaks into the wood. Grease or wax would stay in place better. Or a sheet of slippery plastic.

  5. onslow | | #8

    Another favorite of mine is dielectric grease from the autoparts store. I had HDPE tape leftover from a shop jig project, which is kinda outside easy availability. If the drain line is the culprit, look to the hanger materials. The perforated metal tape should be able to move enough, but worth a check. The plastic J hangers will click a bit if the drain run gets warm enough.

    Sometimes, if one can find the rub point and lift the pipe enough to sneak a bit of fabric between pipe and rub point, that is enough. Adding the dielectric grease into the fabric might become a messy bug catcher, but make the sticking point quieter still.

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