Once I work it out I'll let you know. If people don't think it's damaging it could be months until I find out.

I have located the source of the noise, but not the cause. It sounds like coming from the copper sewage pipe that takes toilet waste.

We had the bathroom redone a while back. So as part of the sewage system under the bathroom we have:

• A metal vent pipe maybe 80mm - 100mm that goes up to the roof and goes under the floor
• A plastic pipe that connects the new toilet location to the old copper pipe
• The old copper pipe that runs under the house

The noise is coming from about a foot past where the old copper pipe joins to the new plastic pipe. It sounds and feels like it's coming from there, but it could be reflected from elsewhere I guess. It happens most often and loudest when the wind is blowing a southerly.

Any ideas? What could be in the pipe / making the pipe make that noise?

If it is in the wind, could it be pressure differential causing the pipe itself to flex? 

I have no idea. I might just get my plumber around to replace that pipe, or at least get his opinion. After the shutdown finishes of course, this noise has been going on for 5 years.

Is it cyclic/recurring, or just random? I suspect it is to do with air blowing across the top of the vent pipe and creating a vacuum. It could be sucking water from a trap or flexing somewhere.

How close to the roofline or other objects is the open end of the vent? Regardless of what causes the noise lower down, you may find moving the vent up/down/sideways in relation to whatever is near it may be enough to change the airflow and stop it happening.

Maybe a pic of the vent?

timmmay:

 

I have no idea. I might just get my plumber around to replace that pipe, or at least get his opinion.

 

Might have to "find" a leak for plumbing call outs to qualify as "essential services"! 😄

Disclaimer: Don't actually make anything leak...

RunningMan:

 

Is it cyclic/recurring, or just random? I suspect it is to do with air blowing across the top of the vent pipe and creating a vacuum. It could be sucking water from a trap or flexing somewhere.

 

How close to the roofline or other objects is the open end of the vent? Regardless of what causes the noise lower down, you may find moving the vent up/down/sideways in relation to whatever is near it may be enough to change the airflow and stop it happening.

 

Maybe a pic of the vent?

 

Sure, I've included them below thanks. Basically, the pipe comes out from under the house and goes up the side of the house, through the eaves and roof, and it goes up about a meter above the roof on the north side of the house. It has one of those little caps on top. Where the pipe comes out from under the house there's a bracket that's not secured one side, but as far as I can tell (I checked) it doesn't move at all there.

Here's where it goes under the house.

Here's the vent on the roof - I had to stand a ways back to get this so it's pretty terrible sorry.

mdf:

 

Might have to "find" a leak for plumbing call outs to qualify as "essential services"! 😄

 

Yeah after the shutdown finishes, and after he isn't busy again. No rush, I created this thread in 2016 ;)

You might actually want to try screwing that bracket down if it is easy to do. Pipes can flex over a surprisingly long distance and it's sometimes only where they join something non/less flexible that you hear the noise. If the bracket is metal on a metal pipe (it looks like it), a plastic sleeve can help cushion the noise. Even a couple of turns of PVC or duct tape might help.

Try changing the pipe dynamics in some way. Clamp it with something or put a rope around it and pull it to create tension. Obviously, don't break it, just change the stresses on it to see if it does anything. You might be surprised.

Which way is south in the photo? Could be the way the air is bowing over the roofline and top of the vent pipe.

Thanks guys. For some reason that existing clamp no longer reaches around the pipe, so I'll have to go to the hardware store in a few weeks to get one, unless by some miracle I have one in the shed. Maybe I can find a way to clamp it down, and under the house it isn't well secured.

Wind is absolutely blowing over the top of the pipe, it's pretty windy here today. It's on the opposite side of the house from the wind, but there's plenty of wind to go around everywhere.

Awesome - good to hear the update!