There‘s always moisture present in the "smoke stream," as a byproduct of combustion. There‘s a plenum, an expansion chamber between the end of the tenon and the floor of the mortise. This expansion results in a combination of turbulence, rapid cooling, and condensation. This is exacerbated by the temperature gradient between the bowl and the stem. So, moisture forms in the plenum. If the tenon inlet is not tapered, the result is increased turbulence at the inlet, which will concentrate the condensate at/near the inlet, hence the gurgle. (If the condensate simply stayed in the plenum, it wouldn‘t gurgle unless you really pulled hard on the thing).
If you "flare" the inlet of the stem, providing a smoother transition, the gurgling will possibly be minimized. You‘ll still get condensation, of course, but more of it will "stay where it belongs."
Prossimo articolo: What about plastic bags, like zip-locks?