Vent Pipe Video
Ever notice that little round pipe on your roof? What's it connected to? Inside the house, this vent pipe is connected to the plumbing system. The vent pipe is connected to the sink drains. This pipe provides make-up air when you drain a sink or bathtub. Since the drain pipes are full of air before you pull the plug, this air has to be replaced as the water pushes the existing air down the drain.
If you notice a gurgle when flushing the toilet, it might be an indication that the vent pipe is clogged. Get up on the roof and put a garden hose down the pipe and turn on the water. Be sure to have someone inside checking for any leaks. The water from the hose hopefully will flush out the blockage and your gurgle will disappear.