When you hear a glub glub or gurgling sound, you are right in thinking it is an airflow problem. Downstream blockages cause the toilet or other fixtures to bubble up, not make sucking noises. You can either snake out your main vent stack or try putting your garden hose down the vent and flushing it with water to blast out whatever is preventing your airflow. One of those high pressure nozzles for your hose would be a bonus if you have one.