Moisture condensing in the airway somewhere. Usually caused by turbulent airflow, if there are transitions in the airway that are not smooth. Sometimes caused by tobacco that is too wet, or smoking too fast.
What does the tenon end of the stem look like? If it's flat, maybe it needs a bevel. Maybe the airway needs to be opened up a little.
Smoking a pipe "well" means you are finding a balance between smoking so slow it goes out, and smoking so fast it gets hot/wet. Water is a product of combustion, and as you heat the pipe up, it boils that water out, as long as you have that balance just so. Tobacco too wet, or smoked too fast (too much water for the pipe to dissipate or absorb) and the equation tips. So when someone says "All pipes smoke the same" that's what they are talking about - there's a user-based component of successful pipe smoking, always.
That said, certain pipes are much, much more prone to NOT handling the moisture well, and one of the things that happens, as JRobert mentioned, is that all the moisture in the smoke (a good thing, it's carrying oils and esters - flavor) condenses somewhere in the pipe and causes gurgling - often at the joint between the tenon and mortise, and often because of misaligned holes or rough edges. This is one of the reasons filters of various design are popular, they are a means of eradicating or controlling this moisture - the Savinelli balsa block is a great example.
So if you have a gurgling pipe, dry your tobacco more, smoke slower, see what happens. Particularly if ALL your pipes gurgle. If you have just ONE particular pipe that is an offender, it's probably some physical thing in how the pipe is built, and we have experts on hand who are willing to assess and help you repair in some cases.
My pipes never gurgle as my pipes smoke dry as a bone.. I make sure the pipes I buy have shank dimensions of 4.0-4.5 shank sizes. My stems are all quality made that are smooth on the inside and create no turbulence. I will not own a pipe that gurgles.
For myself, I can get a briar to gurgle with fresh tobaccos loaded without drying, more often than not. Sometimes if the humidity is high. The cobs never gurgle. Also my technique is often neglectful, puffing too much.
All of the posts above have it covered. However @Ozark Wizard touched on something that is most times the culprit, and that is being neglectful, and puffing too much, or hard. I am guilty of this quite often, and for this reason I want ALL my pipes to pass a pipe cleaner. I never experience gurgle, but kind of catch myself getting a bit "rammy" as I call it. @cigrmaster also made a statement, and I know he loves his Rad Davis pipes. Rad took a lot of extra care on the airways of every pipe he made. I will echo his staement with a little extra words added in--If, and when I know all things on my part are being done correctly--tobacco moisture fairly dry, not going crazy with long, hard fast puffing, and I am sure it isn't me that is the problem, that pipe simply isn't going to be in my cabinet. "I will not own a pipe that gurgles."