Several problems can cause a sink or other plumbing fixture to have a slow drain and to bubble, burp or hiccup. If the problem began occurring recently or got worse over time, chances are the issue is a clogged drain or vent pipe. If the problem always existed or happened after new plumbing was installed, the problem is more likely caused by incorrect plumbing.
A plumbing pipe vent allows air to enter to enter the sink's drainpipe so it drains properly. Without a working vent pipe, fixtures empty slowly and can also bubble, burp or hiccup. Just like your finger over the end of a straw will slow or prevent the water from draining out, a missing or clogged plumbing vent has the same effect on the drainpipe.
A clogged vent pipe can occur if the vent is a "wet vent." In this situation, the vent is also a drain for another fixture. If something clogs the drain, the vent for the first fixture won't work. Main vents can become clogged by snow, ice, leaves and other debris. Clogged vent pipes can be opened by auguring the vent similarly to auguring a clogged drainpipe.
When a new sink is installed where there was never one to begin with, a vent pipe is required for the drain. If a plumber or homeowner takes a shortcut and doesn't install a vent, or if the vent is too far from the drain, the drain may empty slowly. Air bubbling up through the water makes room for the water to drain, which allows gravity to eventually win the battle and the sink basin empties, although quite slowly. The only cure for a sink drain without a vent is to install a vent pipe. Where the drain pipe goes down, the vent pipe goes up and joins with the main vent in the building's attic or topmost ceiling.
Incorrect Drain Plumbing
Water flows downhill. By itself, it cannot flow uphill. A drain that does not slope downhill won't empty easily. The water builds up in the drain as the sink empties and trapped air is pushed up through the trap and out the sink's drain, which may cause the sink to hiccup or burp. Sometimes drains make a gurgling noise as they finally empty. The problem is worse when the drainpipe is sloped in the wrong direction and water has to flow uphill. Again, air is trapped in the pipe as the pipe fills, causing the burping, hiccuping problem. If the sink is above the end of the pipe, gravity eventually wins and the sink empties, but it does so much slower than a drain with a proper slope.
A partially clogged drain can make a sink empty slowly. The problem is an obstruction in the drain pipe that prevents water from flowing freely through the pipe. The obstruction might be a build-up of grease, hair or even just sludge in an old plumbing system. Plunging the drain when the sink is full of water might fix the problem. If that doesn't work, you may need to have the drain augured. After the problem has been fixed, enzyme-based drain cleaners can help keep a problem drain flowing easily.