Bubble of air out of toilet when flushed
If there was a clog in the main drain line going out to the street, you would likely see everything in the house, or at least the basement and ground floor, being backed up or draining slow. More likely is that some animal has set up shop in the vent pipe that serves the toilet. A nest or similar made of twigs and leaves can end up being a pretty effective stopper and prevent air from entering as it's supposed to.