Greek Tortoise Noises while sleeping

by Valerie
(San Diego)

Every so often while my Greek tortoise is sleeping he will make a sqealing noise. I'm curious why, is it normal? Or is there something wrong? He has had a respirtory infection before but he got well and that was 2 years ago. Does it have to do with that? The length of the squeal differs from very short to up to a full second. Any info you have is much appreciated. Thank you.