When its cold the traps are more likely to dry out. Run some water through the faucets to refill them. Its basically standing water that fills the tube so gasses can not go in and out.
This is what a trap looks like. They are ment to have a small amount of water retained to fill the pipe.
It might also be a problem with your plumbing vents.
Once in a blue moon, the vent on my roof gets buried in snow.
It usually thaws quick enough because there’s a bit of warm inside air leaking up, but if we get lots of snow at once, sometimes it happens.
If the vent is plugged and air can’t get in from the vent to allow water out l, it will either start to go uhhh… glug-glug, or sometimes that just siphons one of the other traps to allow air in that way, which then also allows smelly air through there.
You’re likely to not notice the gluglug or the siphoning of another trap happening elsewhere when you flush the toilet.
I’m not a plumber, but that’d be my guess.
Haven’t had any snow accumulation this year (yet), so it shouldn’t be that blocking it. Not to say it couldn’t have a block by something else (it’s almost 70 years old). Haven’t noticed any glugging, but like you said, I may not notice it.
I’ll mention it to the plumber and see what they think.
Air always needs to get in for water to go out.
(Kinda like holding your finger at the end of a straw in a bowl of water doesn’t allow water in or out of the straw)
If the top right vent (roof or wherever) is plugged by snow or ice and you drain somewhere else, it’s likely the basin trap will get siphoned to allow air in the way.
It then doesn’t block sewer gasses from getting back in that way.
My sinks, showers, dishwasher, etc all have a separate PVC sewer line and only the toilet still uses the original steel one. Both sewer lines, PVC and steel, exit the basement separately, but I don’t know if they stay separate all the way to the sewer connection at the street or combine outside. Thanks previous owner /s.
That said, I’ll have to check the vent configuration. I think there’s only one vent and want to say it’s shared between the two, but I have no idea. When I bought the house years ago, the inspector didn’t make any notes about that, so I assumed it was all good.
Thanks for that diagram, btw. Will see how mine compares and hope the plumber doesn’t just recommend ripping everything out and starting over xD
If a vent is lacking the are air admittance valves (AAV).
They’re basically a one way vent for air that allows inside air to be used for venting the pipes but doesn’t allow sewer gasses back up.
It wouldn’t be my first choice as they eventually wear out, but certainly much cheaper than rippong stuff out of the walls.