Most likely, its an easy fix. The first thing I would do is a basic snake to see if it can be cleaned out.
I believe there is a venting problem or missing shower or bathtub pit trap or vanity pit trap.
Maybe your wax ring is not doing its job - pull it out and reset the toilet.
It sounds like a vent or trap problem. Alot of times we get called for a sewer backup and it turns out to be a dry trap. A/C units are great dehumidifiers and have been known to dry traps in floor drains. Simple checks are see if sinks have traps and are wet (have water in the bends of the trap), and check any floor drains. If the floor doesn’t get wet much, pour a gallon of water down the floor drain to ensure a wet trap. If problem still is there, consult a local plumber for a vent/sewer scope.
it could be sewer gas leaking out from your sink … try adding some.vegetable.oil into the drain of the sink and see if that alleviates the issue.