Sounds Is this purring?

[ripshaw]

My guinea pig likes to make this weird noise while eating or just hanging out in his cage. I'm not sure if it's purring or what. He doesn't do it every day, maybe once a week and for an hour or so. Anyone know what it is and why he's doing it? He's a real sweetie, 3yo male that lives alone.

Guinea pig purring? - YouTube (skip to 40 seconds in and it gets loudest)