Knocking Sounds in Drain Pipes

**Lil Baby** · 02-06-2018, 04:39 PM

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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**Lloakley930** · 02-06-2018, 05:19 PM

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”.

**Lil Baby** · 02-06-2018, 05:41 PM

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”.

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.

**Hogster1** · 02-06-2018, 05:42 PM

He said

**Lloakley930** · 02-06-2018, 07:35 PM

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

**Lil Baby** · 02-07-2018, 01:05 AM

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

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.

**Lloakley930** · 02-07-2018, 07:47 AM

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.

**LSL Angler** · 02-07-2018, 09:31 AM

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

**Lil Baby** · 02-07-2018, 12:53 PM

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?

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.

**dfar** · 02-07-2018, 01:09 PM

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.

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