My dog and his favorite toy, why does he whine?

My dog has a favorite toy. It's this soft rainbow-colored squeaky ball. He walks around with it in his mouth all day long. He brings it with him everywhere we go. But he has one odd behavior. He'll sit there and suckle on it for hours at a time. The whole time he's doing it, he whines. He doesn't seem to need anything while this is happening. I try to comfort him, give him snacks, play with him but he'll eventually go back to the ball and start whining again.