This is only a theory I have, based on limited observation (just my own piggies), but I think piggies learn language mostly from other piggies while they are growing up, not all of it in the first few weeks as babies either.
They learn another piggies 'happy' noise first and then develop it into their own when they are babies learning from their adopted Mum while being cuddled. It is fascinating hearing the echoes of former piggys in the contented chuckles of current piggys who have been the former piggys 'babies' in the past.
If there is a noisy piggy in charge of the herd, or one who is happy to let rip every now and then anyway, then they all join in the wheeking but if the leader is a quiet piggy then the whole herd quiets down and the wheeking gets infrequent and can even stop altogether.
A piggy when they are a baby will make noises constantly when on the move. 'I'm over here' noises that I used to call 'squeaky booting' because they made a noise whenver they moved a foot along. This also fades if there is a quiet piggy leader.
Piggies also learn how to rumblestrut from others and how to make noise when they want something. Because of the varying environments that your piggies can start life in they will come to you with varying levels of vocalisation and also various levels of learned communication. This means that one noise in one piggy can mean something else to another sometimes. Taking the example of teeth chattering is a good one - in a piggy that hasn't been with piggy kind this can be an 'I want' noise but to piggys who have learned the language this is usually a 'go away' noise when done loudly, so this can lead to real language problems when introducing piggies. The newbie who doesn't know any better is basically shouting 'go away' at the established herd, while trying to say 'I want to join in'.
Libertychance; What I am trying to say is that the body language is a better sign sometimes than the noises, especially if your piggies haven't had the chance to learn all the piggy language from a herd of piggies. So look for 'happy ears' all perked up and listening. Look for relaxed bodies with the spines all softly relaxed and the tummies all settling onto your lap. Look for legs stretched out and the back legs even gently paddling into comfy positions. Look for piggy sighs, those short deep breaths of real contentment. Don't rely on the noises, look at the body language as that is by far the best indicator of whether your piggies are happy or not. Once you understand from the bodies how your piggies are feeling you will be able to translate their version of piggy language.
Happy cuddling!