Guinea pigs only chatter their teeth whenever something is wrong or something has upset them. If your guinea pig is currently chattering its teeth it means that there’s something going on in its life, and it’s up to you to figure out what it is! Sometimes it can be a new product like a phone or a camera, and sometimes it can just be upset with the owner in general. No matter what it is you can rest assured knowing that guinea pigs chattering their teeth is a sign of being upset/angry in one way or another (keep that in mind!).
Here is an example of a guinea pig chattering its teeth.