This is an often asked question and it's all about physics.
The first picture below shows the general construction of a clearomizer.
Note: it's a bit different for Bottom coils, if you have a bottom coil go to the END of this page.
You have a base, attached to the base is a cylinder, the cylinder is empty it is empty for airflow, air is essential in getting a proper vape, no air, no vape. Attached to the top of the cylinder is a heating coil with wick strung through it. Above the coil and wick there is usually a liquid sheath (called a condom by many) this sheath protects the inside of the cylinder from getting liquid in it. The concept is that the wicks will wick the liquid up to the coil and it will not get into the inside of the cylinder. Most of the time, this works well.
However, wicks are both your best friend and your worst enemy.
Below is a "normal" view of a clearomizer, the liquid is being wicked up to the coil and there is nothing in the center cylinder.
Gurgling is caused when you have liquid trapped in the cylinder and it mixes with air and gives you the gurgle sound, like sucking on a straw when there isn't much soda left in your cup. Ask a kid, they know all about this :)
Gurgling can happen when liquid is trapped in the cylinder or when there is excess liquid in the coil area.
Now to understand how it gurgles you need to get into a bit of physics, because I am not a physics teacher I'll make some assumptions and cut it short. Water attracts water, by that I mean, once you have created a stream the water will follow. eLiquid is no different, in fact due to the thicker viscosity it will "flow" more easily as eliquid doesn't just evaporate and it's "sticky" so it will stubbornly stay near the lip and on the sides on the inside cylinder.
If you tip your clearomizer upside down, there is a very good chance you will get liquid into the coil area, more than you want and some of that liquid if not burned off quickly will drip into the Cylinder and down the inside. Because there is now liquid inside and on the lip of the cylinder, some liquid coming up from the wick will syphon off and "flow" toward the other liquid, resulting in drier hits (as less liquid gets to the coil) and gurgling as the air mixes with the inner cylinder liquid.
It's simple yet complicated at the same time.
here is another crude drawing of what happens..
The bottom line is that once you get liquid into your center post/cylinder you're hosed.
You need to get it out.
To do this you have several options, the first and most complete is to rebuild it, if you have a rebuildable clearomizer like the Thor, Vision, Kanger etc, it's an easy thing to remove it and get the liquid out of the center post, just flush it out and let it dry.
If you do not have a rebuildable, like a CE4 V2, then getting it out is trickier and doesn't always work. The easiest method is to blow out the liquid with a reverse vape, just blow into the drip tip with the clearomzier sitting on a paper towel (to catch the excess), do this a few times, turn it over, repeat and repeat again until nothing else comes out. Then turn it upside down and let it sit overnight with the drip tip off (again on a paper towel).
This will hopefully remove all the trapped liquid in the center cylinder allowing you to get back to normal.
This method also works well for flooded tanks and other various issues that come up while vaping.. when in doubt... blow.
Bottom Coil clearomizers leaking?
If your bottom coil is gurgling then it would the same as above there might be liquid in the center tube or seeping into the center tube, use the same blow out procedure to clear it.
If your bottom coil clearomizer is leaking then there are a few things to check:
1. The base and coil are on nice and tight and straight. In some cases a coil can go on slightly crooked, this can cause it to leak, remove the coil, twist it on, it should take barely any effort to get a coil on a base.
2. Check to make sure the oring seals are good (see pics below)
3. Next make sure the base is on properly and there is no gap between the base edge and the clearomizer.
Images (not all bottom coils are the same but these are images of what you should look for.
1. Check the coil Orings
2. Check the coil is tight and properly on the base
3. Check the base orings
3. Check BOTH orings
4. Make sure it is tight to the clearomizer, no gaps.
One last thing, keep in mind though that this kind of problem can always be caused by shoddy design and workmanship on the clearomizer itself or a torn or cut sheath, it's not always going to be your fault. Sometimes you just get a bad one :(
However due to the ease at which someone can ruin an oring by over tightening, not putting a coil on properly they are not warranteed for this, we generally carry bases for bad seals and orings for purchasing when it happens.
If your clearomizers worked initially and now it's leaking, it's most likely a seal or simply a bad coil.
In some rare cases leaking can actually be caused by the battery connection or even your liquid choice!
Because different manufacturers have different tolerances there could be as much as 1mm difference betwen your battery threding and the clearomizer you have purchased. This extra space can push UP the coil or center pin of your clearomizer causing just enough room to start leaking. Check this by screwing your clearomizer on your battery and then backing off a half a turn. In some cases this can stop leaking immediately.
In more rare cases the liquid and voltage/ohms you are using can be a factor due to excess heat and or how the liquid reacts to heat, getting thinner thus allowing leaking where there might not have been any before. If you want to test your cleaomizer/liquid for this stop vaping, clean the clearomizer and connection and then take one vape and leaving it for 30 seconds and repeat for a while, if it has stopped leaking, it is your voltage/ohms and or liquid combination.