It actually means displeasure, despite what everyone else thinks. It isn't even a peacefull noise, and when I hear it I get a sence to back off. A purr is like a soft, quite wheek type sound, though it is not a squeal.
The noise the pig was making can come from when they are in displeasure from either touching, or even an obnoxious ugly sound.
YouTube - Norton Purrs
the piggie in the clip is purring in the beginning, though not when he starts petting near his rump and makes that sound of displeasure again. Near the end, you can hear the piggie start to get a little frustraded that he continues to pet him there. It took me a while to find this, cause everybody think the sound of displeasure was a purr, and ti drives me nuts! haha.