This is hard to explain.. But I was sitting in my bedroom when all of a sudden my toilet started draining, then the sinks in the bathroom started pulling air in, then when the water from the toilet drained, it started suctioning air down the toilet too. It all lasted about 20 to 30 seconds. This apparently happened a couple of weeks ago as well according to my wife.

This only happens in our new addition to the house, which has a partially separate line to the sewer (it joins up to the main line about 15 feet in front of the house). I didn't see any sign that it affected other parts of the house on the original waste water line.

My only guess is that the suction is coming from all of the water in the line draining into the sewer at once, pulling air with it until the sewer line is open to the air.

We have a tub, a toilet, a shower and two sinks on this line and none of them were in use at the time. I was seriously considering that someone was working on the sewer line outside with a vacuum or something. Any ideas what might be happening here?

UPDATED WITH PHOTOS: Here you can see multiple vent pipes. I snaked both the close one and the far one. Time will tell if it worked