I can answer this question for you. They do make popping clicking sounds. Sort of like communicating. They use these for a variety of reasons. It can be the start of a threat, but is usually to voice slight discomfort. They will also make these sounds directed at you, but most of the time not in a hostile way. If they're in a rapid repeatative form then they're about to growl and threaten you.
Mine will sit and make these sounds while i bathe them or am feeding them. I do not know the full nature of these sounds because as far as I know there haven't been any studies on them. You can see when they make the sound if they choose to make them quietly by watching the frog's throat and sides. It will look almost like a stutter or twitch in their throat and sides while breathing. It can be seen in the water as well while they're soaking. They can make these sounds at many different volumes so listen carefully. It's a very interesting and curious sound. I assume tbat it is used to communicate with other Pacman frogs as i've had mine make these clicking popping sounds back and forth to one another a few time. Wish there was more of a study done about this subject.