Gunther does this as well, and it drives me nuts. He will stop when I look at him b/c he knows I dont' like it, but as soon as I turn my head, I can hear the slurping noises again. (sorry slurping noises drive me up the wall). He started doing this right before he got a cold, and has been doing it off and on ever since. I've always contributed it to nasal discharge or post nasal drip, but I don't know if that is what it is or not.
Keiko did this too when she first got allergies, which is why I thought the same for Gunther with the post nasal thing.