A sink's drain normally does not let bubbles out, even when you are draining water from the sink. The presence of bubbles in the sink's drainpipes indicates a problem with an obstruction in the pipes or the sink's venting system. You do not need to necessarily call a plumber to correct the problem.
Sometimes a sink's drainpipes develop a partial clog, or an obstruction in the pipes that still allows some water to drain down the pipes. The obstruction may be formed by hair, soap and other debris washed down the sink's drain, which collects inside the pipe. As the obstruction forms, it begins to trap some air behind it as water flows past. This air bubble behind the clog may release tiny amounts of air at times when you drain water from the sink, leading to bubbles coming out of the sink's drain opening.
Removing the Clog
If you see bubbles coming from your sink's drain, the first step you should take is to remove the sink's drain cover or stopper. Some partial clogs cling to the stopper or drain cover, making them easy to remove. Also use a plunger to apply pressure to the sink's drainpipes, which may force the partial clog free. If you continue to see bubbles coming from the sink's drain, you must take the pipes apart and check for clogs in the different sections of pipe. Feed a drain auger down the pipe that goes into the wall, eliminating partial clogs further down the drain pipe as a potential cause of the bubbles.
Clearing clogs from the sink's drain line may not eliminate the bubbles, indicating the problem lies with the sink's venting. The sink's drain line attaches to a larger pipe, called a branch drainpipe, which also receives water from other nearby plumbing fixtures. The branch drainpipe connects to a vent pipe as well. The vent pipe allows sewer gases trapped in the pipe an escape route out of the house through the vent pipe's opening above the roof. Vent pipes also replace any air washed down the pipes with the water. An obstruction in the vent pipe for the sink may lead to bubbles coming through the sink's drain.
Clearing Vent Pipes
Since the vent pipes go through the house's roof, you must climb onto the roof to remove any clogs. The sink's vent pipe should be located approximately over the room where the sink sits. Sometimes you may find an obvious obstruction such as a bird nest or tennis ball at the opening of the pipe. Other times you must look down the pipe with a flashlight to locate the clog. You may use a drain auger's flexible line to push the clog out of the pipe. A garden hose with a high-pressure attachment will also knock loose clogs deep in the vent pipe.