Knocking Sounds in Drain Pipes

Once again I turn to the experts at CDC. The two pipes below drain the upstairs master bath and spare bath. They make a real annoying knocking sound ONLY when water is draining. They are PVC pipes that seem very secure. I've ran this by several buddies and have not got a definitive answer. I really don't want to call a plumber if this is an easy fix. Closest thing I've seen on YouTube is heat expanding and contracting in the pipes but it makes the noise when cold water is running as well. Handyman buddy says there is air in the pipe that can be taken out by some kind of respirator but the noise won't hurt anything. I tried to upload the noise but can't some reason.
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Look under the sinks up there and see if you have a plastic mushroom looking piece. That is the vent on each bathroom. If it’s bad then that’s where you find them. They are called studor vents. If you don’t have them then your vent goes up through your roof.

If it’s water pressure on the water lines then it will only happen right when water is turned off or on. That would be called “hammering”.

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Originally Posted by Lloakley930

Look under the sinks up there and see if you have a plastic mushroom looking piece. That is the vent on each bathroom. If it’s bad then that’s where you find them. They are called studor vents. If you don’t have them then your vent goes up through your roof.

If it’s water pressure on the water lines then it will only happen right when water is turned off or on. That would be called “hammering”.

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My plumbing is on a septic and vents through the roof. This only happens when water is draining and for a few minutes after there will be a few pops. It's more like popping sound.

He said :ThumbsUp

Might have something clogged. Isolate which plumbing item causes the popping. Most likely a sink is partly clogging the vent behind it

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Originally Posted by Lloakley930

Might have something clogged. Isolate which plumbing item causes the popping. Most likely a sink is partly clogging the vent behind it

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Doesn't matter if the tub or sink is running. Now it does have to run for a little bit before the noise starts. The toilet flushing will not typically cause any noise.

You can buy a snake less than 20 ft for sinks and shower. You may be able to unscrew shower grate and snake it.

For the sink, stick a bucket under there and use channel locks or hands and unscrew plastic nuts on the p trap. Drain water and gunk into bucket. Snake pipe going into the wall. Could be hair building up over time clogging the venting ability.

Was this a new house and has it been doing this since you moved in or has it just started?

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Originally Posted by LSL Angler

Was this a new house and has it been doing this since you moved in or has it just started?

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The house was built in the mid 90's. We've been here about three years and it's always made noise but it's getting louder so I'm noticing it more. I thought it was a loose pipe at first and kept letting it go. Finally I cut the wall to see what it going on and it's definitely not a loose pipe. Luckily I've just had to cut a small hole in the closet so far.

I am wondering if it might be caused by a backflow valve in the mainline of the septic. I would cable the drains like Lloakley said anyway first.
With the age of the house a partially clogged drain is most likely the issue. Also double check the vent pipe for a pop vent even though it has a roof vent.

https://www.youtube.com/watch?v=dqDM3WS2vFw

This is the closet thing I have found on the web and is very close to what my pipes sound like.

Wow never heard of that. I guess now test with cold water only and hot water only.

It will make the noise with cold water only but not near as much. If you run cold for a bit then hot it will get fairly loud. I'm thinking this video shows what is going on...it's very weird...and I might run it by a plumber just to get a professional opinion...that's not to undermine the vast knowledge of CDC though....

You may need The Exorcist.

I would say depending on how warm you keep your house the pvc may still pop with cold water. Is your heating ducts near or right next to your pvc. Anyway your pip is probably rubbing on something when it expands and contracts. I have a sink in my basement that does this.

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The pipe may be tight going through the wood to the point that you are hearing this expansion and contracting of the pipe.

I have had to repair this problem several times in new homes. It is the pipes expanding lenghtwise (getting longer and shorter) as the water heats and cools the pipes. Usually hot water causes the most problems. The pipe will pop rapidly as it gets hot and then pop slowly as it cools when the water is shut off. It may take an hour for the pipe to cool. The problem usually occurs where the pipe penetrates through the top or bottom plate(2x4) of the wall and is caused by the pipe rubbing on the plate or a metal fire stop around the pipe if used. You will have to open the wall and cut the hole around the pipe looser to allow it to move. Sometimes you can just slip a piece of card stock or thin cardboard (cereal box) between the pipe and the plate and this will allow the pipe to move without binding.It's a PITA but it's better than listening to the pipes pop for an hour when you are trying to go to sleep. Good Luck.

PVC POP!!! Ive had it in our condo for 25 yrs, Plumbers wont touch it, say its not hurting anything, no leaks etc. Live with it. You used PVC instead of solid metal.

Thanks for the replies I'm convinced now that it is just expanding and contracting. I might try to stick some cardboard or something around the pipe to see if that helps. I was more concerned than annoyed. If this is my biggest problem in the house then I'm doing pretty good, knock on wood. My last house was pretty old and had copper pipes. In the garage the pipe that drained the kitchen sink wore through and leaked. Nothing J.B. Welding didn't fix.

Do the pipes run through an area that is exposed to cold air. If so maybe wrap the pipes with insulation to minimize the temp change.


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