My opinion on the matter, has strictly come from experience with boys... My boy pigs would make that sound and then the fighting would start, I think its a territorial thing. I could be completely wrong, but I found with my boys that was mostly it. It stopped when we separated them. When they were happy they never made that sound, just content little squeaks. And when i stopped petting one he was notorious for nudging my hand onto his head so he could get more ear-scratches... It could be completely different with females, or as stated above, high and low pitch gurgling sounds could mean different things.
Mabye its like humans saying "Hey" it could mean "HEY! NICE TO SEE YOU!
" or "Hey... put down the carrot and back away from the food bowl"
