Solution to a bubbling toilet and sudden water level changes in the toilet bowl. Both indicate an air-venting problem in the drain system.
If you have air bubbles that rise up through your toilet bowl (except when it flushes) or notice the water level rising and falling, you probably have a clogged or improperly vented toilet. This is especially true when you have an appliance like a clothes washer nearby. Your drain line is gasping for air.
When you pour liquid from a can, you’ll notice that it doesn’t flow evenly unless you have a second opening for air. The same holds true for plumbing. As water goes down a drain, air is needed to equalize the pressure in the drain line.
This is the purpose of a venting system. If the drain lines in your home have poor venting (as in Figure A, for example), water rushing down the drains will pull water from nearby P-traps. The drain in the toilet bowl is basically a P-trap. If the problem just started, it’s probably a blocked drain or vent that needs to be “snaked” out. And since the water in the toilet is dropping and gurgling, it’s likely that the problem is near that area.
Unfortunately, a clogged or missing vent is tough to fix, since it usually requires breaking into the walls to examine the drain system. Unless you have plumbing experience, this project is best left to a professional.