Have you ever taken a shower and noticed that your toilet starts to bubble? It is not an uncommon problem, and the reason for it is usually a blockage somewhere in your plumbing system. The reason for this is that all the pipes in your home are connected, and when water goes down a drain, it will push air out of the way. If there is no way for it to vent properly, it will find the easiest way out, which will many times prove to be your toilet. You should have a vent pipe connected to your plumbing, and if this got plugged by a birds nest or something similar, this might be the reason why you are having problems both with air venting through your toilet, as well as, slow draining throughout the home.
If you can access your vent pipe, checking it for blockages and clearing it out will most likely solve the problem. Just be careful that no critters get startled when you go to remove the plug, since their only way out might be right past your face.