Vent Stack comes a-knocking

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Matt1984

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Sep 14, 2019
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Hello,

we’ve been hearing some knocking (and slight creaking) in our wall behind our toilet during the evenings for the last week or two. My wife also thought the bathroom smelt a little “sewagy”.

I cut open the wall today and noticed the vent stack had a crack in it at a tee. This explains the smell (and we’ll fix it), but I’m not sure if this crack is the source of the noise though. It mostly seems to occur when water is draining into this stack from the kitchen below, and the sound doesn’t seem to be coming from the crack itself.

I have a read a few different things about the noise: mostly that this is normal with the cold air from outside and the expansion from the heated water, or that the stack is rubbing up against wood and that cutting away some of the wood would stop it.

This stack/vent pipe seems to be clear of wood, but it is a pretty wide pipe and is very tight against the drywall. I attached a picture to show how the drywall is actually being pushed off the stud by the pipe (it also shows the crack in the tee). Also, the drywall I removed had marks from the fitting on the back of it, indicating it was rubbing up against it too.

I guess my general question, which sounds pretty obvious as I’m asking this, but does it have to be tight against wood for this noise to occur or would drywall have the same effect?

Also, would insulating this wall help reduce the noise at all? This vent is in a wall that runs perpendicular off the outside wall and is less than a foot away from it.

Finally (and just for my own curiosity), is this vent stack normally this thick? It seems like this would be a problem everywhere if it is (unless they normally use 2x6’s or something for this wall).

Thanks!

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