Well, in my experience, its a happy sound. They're contended, bit like cat/ I've also noticed that its a kind of "comforting" noise to make themselves feel a bit better when perhaps they're a little anxious. I own three boys and they all make the noise - especially if you rub their behinds (think this has more to do with their winkies waking up than the it actually being me touching them!) ")
Its a good thing by any standard!
(Unless more experienced owners disagree ...?)