I've got a protank II and have had it gurgle on me. I've followed the instructions on Kanger's own site which reads: Gurgling noise or leaks: This is a sign that liquid has found its way into the metal tube in the center of the Clearomizer (and usually occurs after a refill). Ridding the center tube of liquid will resolve the issue. To do so without emptying the tank, remove the Clearomizer from the battery, wrap a cloth or tissue around the battery end of the Clearomizer and blow through the mouthpiece.
Guess what happens? It drains the entire contents of my tank onto the tissue. Then, the gurgling stops. Glad I could solve the problem by sacrificing a half-days worth of liquid. Grrrrrr...