Bathroom toilet has air bubbles when I shower
Sounds to me like a roof vent is plugged or restricted. Run water from a hose into each roof vent and see if you can clear them. Have someone inside while running water into the vents to insure that if this would cause a water leak inside that you stop before doing damage.

Or run a snake down each roof vent. But don't drive the snake into the drain joint where the vent connects to the horizontal runs. You could damage the drain and turn a minor problem into a major one.