drain-waste-ventplumbing
Recently, the drain in my second floor tub makes a gurgling sound when I flush the toilet, which is located right next to the tub. They are obviously on the same drain line. I know that there is a vent line going up from the bathroom through the attic, but I am not sure of the exact configuration.
In all other respects, both toilet and tub seem to drain normally.
What could be causing the gurgling?
You must have a vent. The toilet will not flush properly without one, and there will rarely be enough water left in the bowl. You need a 2" vent to take off from the branch line within 6 feet of the toilet. It can run up any convenient wall until it is above the toilet's flood level. Then it can run near horizontally (1/4" per foot) anywhere to tie into an equal or larger vent.
There's any number of ways to properly vent a toilet, and several common ways that are technically wrong, and even more that are just plain wrong. It's impossible to suggest a good method without a lot more information.
Sounds like the main drain for the bathroom may be restricted.
As you shower the water doesn't drain away fast enough, and the plumbing begins to fill with water. As the pipes fill, air may be forced out through the toilet trap causing "bubbles". Since the tub is the lowest drain, the water backs up there first. If you continued to shower, you'd eventually see the toilet overflow as well (if the tub edge is higher than the toilet rim). If the tub was deep enough, you'd eventually see the sink overflow too.
If you look at this crude representation of your bathroom, you'll be able to see what I mean.
Now, if we add in a clog we can start to see what happens when you shower.
As you shower the water cannot pass through the clog fast enough, and the pipes begin to fill.
At this point, any air trapped in the line between the drain and the toilet trap will be forced out and bubble up trough the trap.
As you continue to shower, the tub is the first to back up.
Eventually, if you shower long enough the toilet could also overflow.
Depending on the height of the sink, quite a bit of the bathroom would have to fill before it overflowed.
The first thing I'd try is to clear the drain using a Plumber's snake. A hand auger would likely work, and can be purchased at a local hardware store fairly cheaply. Drill powered augers are also available for a bit higher price, but require a bit more of a gentle touch (and obviously a drill).
Another option is that the vent for the bathroom is clogged, or not sized appropriately. If vent stacks are not covered properly, leaves and other debris can become lodged in the pipe and prevent air flow. A clogged vent will cause the fixtures it's venting to drain slowly, and the results can be similar to a clog as described above.
As with the clog, the solution to clear the blocked vent is a Plumber's snake.
Best Answer
Two options. Air is going in, or air is coming out.
"Gurgle" is caused by air being forced through the water in a fixture's trap. It's similar to the "glug" generated when you pour milk quickly from the milk jug. It's air being forced through liquid, in an attempt to equalize pressures.
Air in
Vents placed strategically throughout the plumbing system, typically provide an adequate amount of air. If the vent(s) get clogged/blocked/restricted, pressure differences inside and outside the system will develop. If these differences in pressure are large enough, air will be forced through fixture traps in an attempt to equalize the pressure.
In this case, clearing vents; or adding additional vents in the case of improper venting, will usually resolve the issue.
Air out
If a large enough object (liquid and solid waste) is moved through the system fast enough; in such a way that the object blocks the entire internal pipe diameter, a high pressure zone can be generated on the front side of the object. This high pressure zone may be able to force air out through fixture traps, as the object travels through the system. Think of it like the plunger of a pop gun.
In this case the solution is to increase the diameter of the pipe, eat less fiber, use less toilet tissue, or decrease the flush power of the toilet.