Why do guinea pigs vibrate when you hold them?

When you are petting your guinea pig, and it makes a noise, that's called purring (like cats but a lower volume) and then their body vibrates, that means it's content with however you are petting them.

I have a guinea pig named Chelsea (just got her today) and she does this every time I hold her. It's adorable!

It is like purring which is actually a strong vibration of their heart because it beats faster than ours does.