Strange suction in toilet and sink, like a vacuum in the sewer pulling the water out
Even if someone was &quot;working on the sewer lines with a vacuum&quot; properly sized and functional vents would keep the traps from siphoning.
So, you have vents which are not functioning as they should, whether from being built improperly in the &quot;new addition&quot; (since the problem is limited to there) or from some sort of blockage (animal nest in the vent pipe or the like.)
You, or, if that's not your thing, someone you hire should get up on the roof and check the vent pipe there, perhaps with a snake in hand to try cleaning it. Dumping a bucket of water down, or running water from a hose down can also sometimes be helpful in identifying a blockage (and can sometimes help to clear it.)
After that you (or someone you hire if it's not your thing) would get to checking and using all the cleanouts down inside the house, if the problem persists. For a typical venting setup with a single toilet, the fact that the toilet is affected points to the main vent (for this section of the house) being blocked, not just some tributary branch vent.