Gurgling in the drain is never a good sign. Fortunately, the problem is typically easily repaired once the cause is determined.
1. Drain Clog
The most common reason for gurgling in the drain is a localized drain clog. This means the drain has a developing clog in the drainline that feeds only the drain making the noise. For example, there could be a hair blockage in the bathroom sink drain or food stuck in the kitchen drain. At this point, the drain still empties, but the water is gurgling as it forces its way around the developing clog. The drain will eventually back up if the clog isn't cleared.
2. Sewer Line Issues
Sometimes the root cause is a more severe clog -- one in the main sewer line. In this case, you may hear gurgling from more than one drain in the house. The cause is similar to a localized drain clog. Water is making the noise as it drains around a developing clog. Gurgling can also be the result of sewer gases coming up the main line and into your home.
3. Blocked Vent
There are plumbing vents on the roof of your home, which are designed to maintain a balance of air in the pipes so that the drains can flow as they should. If a vent becomes blocked by debris, such as a damaged vent cap or a nesting bird, the lack of air slows drainage and gurgling sounds can be the result. Repairing or cleaning out the vent will solve the issue.
4. Leaking Pipe
Although less common, sometimes a gurgling noise is actually the result of a leaking pipe. The gurgle isn't the sound of water trying to get around an obstruction but of it forcing its way out of a hole or crack in the pipe. You may be able to spot a leak easily if it is right under the sink, but it may not be as obvious if the leak is hidden in the wall.
5. Dry P-Trap
The P-trap is the curved pipe under a drain. Its job is to hold a small amount of water, which acts as a barrier to prevent sewer gas from coming up the drain pipes in your home. P-traps can go dry if a sink isn't used regularly, such as in the guest bathroom. When this happens, you may hear gurgling in the drain as the gas makes its way through the trap. Running the tap moistens the P-trap and solves the issue.
Contact a plumber immediately if you have a drain that is making a gurgling sound.