Sick High pitched squeak every so often?

Every so often my guinea pig has been making mysterious high pitched whines. It sounds like he's in pain. I took him to his usual vet and the vet checked him over and said he looked okay, no injuries. I'm worried about him. I've checked him over several times and nothing looks wrong to me either. He seems to be going to the bathroom normally. He usually makes the noise when he's sitting in the little hay hut I have for him, but occasionally he's made the noise out in the open. He does it several times a day. Anyone have any experience with something like this?