I've been noticing an occasional sewer smell in my bathroom, and finally realized it's only present when it's below freezing outside. As soon as the weather warms up, the smell goes away. For reference, my house was built in '55 and the toilet is plumbed into the original steel sewer line. There are no leaks (everything below the upstairs bathroom is easily accessible from the basement), and no issues flushing. House has sewer service, so no septic tank to worry about.

Could it just be the steel pipe contracting in the cold? If so, would replacing the wax seal address the issue?

Additionally, if that's plausible, would putting a space heater near the steel pipe help by warming it back up (temporary solution to address the smell). Basement is finished and conditioned, but I don't heat it for comfort unless we're spending time down there. I'll just deal with it being 60 degrees when I'm doing laundry.

If not, any other ideas? That's my only hypothesis, so hoping to get some feedback.

I've got no problem calling in a plumber and plan to do so, but I'd just like to have an idea of what I'm up against so I can set expectations for pricing and such.

Edit: Have ruled out dry drain traps. Current plan is to just let the plumber figure it out and hope for the best. Thanks for the suggestions.