Could be purring, which is a sign of a happy and content pig. Vibrating can also be a sign that the pig is cold or in pain -- so just keep an eye on him. You will learn with time what the noises your pig makes means!
Just google "guinea pig noises" and some sites will come up, but in the meantime here are a couple sites to get you going:
Guinea Pig Sounds or
Guinea Pig Sounds