You expect to hear gurgling sounds from the toilet when you flush it -- the noise provides a reassuring sign that everything is working properly. Gurgling sounds from a nearby fixture, such as the bathtub, aren't normal, though, and they usually indicate a blockage in the vent line. If you hear the sounds only in winter, the blockage is probably caused by ice.
Why the Gurgling Is Bad
All the plumbing fixtures in the house are connected to the main vent stack. The vents allow air into the pipes to equalize pressure when water is flowing, and without them, suction draws air through any available opening, including sink, bathtub and toilet P-traps. The gurgling sound you hear from the bathtub is the sound of air being sucked into the drain and passing through the water in the trap. If you don't do something about it, the water in the P-trap may drain out, and you'll start to smell sewer odors.
Causes of Blocked Vents
The vents all connect to the main stack that passes through the roof, and most blockages occur there. It's common for the stack to get blocked by snow and ice in the winter, especially if it's constructed with 2-inch-diameter pipe, as many stacks are. This is called ice capping, and it's an especially pernicious problem in places that get a lot of cold, blowing snow. If you notice gurgling in warmer months, the culprit could be leaves, sticks or even a bird or small animal. It's also possible for debris or critters to get inside the vent -- usually at a bend in the pipe.
What to Do
If your bathtub gurgles in the summer months, it may take only a quick trip to the roof to clear whatever is blocking the vent. If no blockage is visible, spray water into the vent opening with a garden hose. That may be enough to clear the vent, but the water may also back up and overflow. In that case, it may take a sewer auger to clear the vent -- and that's usually a job for a plumber.
It isn't safe to climb on your roof in the winter, but ice capping is usually best handled from the attic anyway. A short-term solution is to locate the vent pipe and use a hair dryer to warm the section closest to the roof to melt the ice. Wrapping the pipe with insulation is a long-term strategy that may prevent the formation of ice. If you have a 1 1/2- or 2-inch vent pipe, you should retrofit the stack opening with a wider 3-inch pipe. You may also consider wrapping the event with heat tape connected to an electric circuit in the attic.
It isn't common, but in some cases, the vent blockage may be caused by an obstruction in the toilet waste line. It's always a good idea to plunge the toilet to see if that stops the gurgling.