Similar to cats, some guinea pigs purr when they are pleased or content. You may hear this vocalization from your cavy when they are being petted or during cuddle time. Some pigs will also purr after being given one of their favorite foods.
Truffle occasionally purrs at the start of lap time, shortly before settling down for a snooze. She'll even purr back at me if I mimic her, like in the video above. But her loudest purrs are saved for when she is returned to the comfort of her cage. Diva pig!
What makes your piggies purr?