Toilets make odd noises, such as gurgles and burps, due to the movement of air through water. Most often, the air movement responsible for such noises is due to a clogged or improperly working vent stack. These clogs cause a vacuum to form as water drains. Once the force of gravity empties the water, the vacuum pulls air through the trap.
The proper name for the drain system in a home is the drain-waste-vent system. The nomenclature emphasizes the fact that the system's vents are as important as its function at waste removal and draining water. Vents help to dissipate sewer gases but also function to balance the air in the system. Plumbing codes require every drain and toilet in the home connect to an upward-sloping pipe that connects to one of the home's main vents. If the vent system is not working correctly, fixtures make odd noises.
While cleaning a vent is normally a simple procedure, it involves working on the roof several meters above the ground. Homeowners who are not comfortable working in these conditions should hire a plumber. The plumber normally uses a plumber's snake or water pressure to clear drains. Once they are working correctly, the strange noises normally end.