Re: Not sure if this goes in plumbing
I agree, too, that it's a venting problem. But you indicate that it only happens in very cold weather.
I suspect that there is a horizontal run of a vent pipe (possibly in the attic) that is not sloped correctly. As a result, rain water entering the roof vent collects in the pipe, blocking the free flow of air. During the summer months, with less rainfall, it dries out. In moderate temperatures, that water in the pipe is liquid and may allow it to 'gurgle' there instead of the traps under the sinks; in extremely cold weather it freezes and totally blocks airflow.
Repair may be as simple as properly supporting the pipe to maintain slope so that the water can drain down all the way to the septic.
Another possibility: the snow on your roof is higher than the vent pipe, and the top of the pipe is blocked with snow.
The "Senior Member" designation under my name doesn't mean I know a lot, it just means I talk a lot.I've been a DIYer since I was 12 (thanks, Dad!). I have read several books on various home improvement topics. I do not have any current code books I can refer to. I was an apprentice plumber for two years.