A bubbling toilet bowl is a sign of either a blocked venting system or obstructed drain pipes. An obstruction may allow the drain pipes to fill with water, forcing air to escape through the toilet bowl. Low flow toilets use a maximum of 1.6 gallons per flush. With such a small amount of water flowing down the drain, an obstruction is not easily washed away. The problem may get worse if ignored, so it is important to diagnose the cause as quickly as possible.
Plunge the toilet. If there is a blockage in the toilet or the toilet drain pipe, the suction from the plunger may be enough to dislodge it. Flush the toilet while running faucets in the bathroom to see if there is a blockage in the drainage line. Watch the fixtures drain to determine if the toilet bubbles more when other fixtures are draining. Also check to see if any or all of the other fixtures drain slowly. If not, the problem can be isolated to the toilet and the drain leading to it.
Run other faucets in the house. Flush the toilet and watch to see if the toilet bubbles more when a specific part of the drainage system is used. Also check if the fixtures drain slowly. If other fixtures are draining slowly or backing up, you may have a clog or damage blocking the main sewer drain from the house or, if you're on a septic system, the tank could be full or the system otherwise obstructed. Contact a plumbing professional to diagnose the problem.
Check the main vent stack by using a ladder to climb onto the roof. If an animal or bird built a nest inside the vent pipe, it will cause a vacuum, slow or stop your drains, and cause the toilet to bubble. Ice or snow may also form inside the vent pipe, although this is unlikely due to the heat of the system venting through. If the vent is obstructed, running water from a garden hose down the pipe will clear it. Because climbing onto a roof can be dangerous, it's recommended that you hire a professional to do this.