Someone once posted a link to security cam footage that picked up their hedgehog's nightmares. Here's the link:
Does that sound like what you're hearing?
Someone else was recently startled by their hedgehog making sounds like fighting cats while sound asleep. My little guy has been chirping in his afternoon naps recently.
Welcome to the forum! It sounds like you do a lot of research on our tiny friends, so you've probably already encountered this, but if not, here's an awesome care-guide.
Also, if you still have his x-ray, do you mind posting it? I've seen lots of x-rays of unhealthy hedgehogs, but very few of healthy ones, and I'd love to get a broader understanding of what "normal" looks like.