Worried about 9 month old crying in sleep?

My baby is 9 months and has slept peacefully only waking once in the night for a couple months now.

Tonight it’s almost 11pm, he’s been asleep since 8, and I can’t sleep because every 30min to 1hr he cries out and makes the saddest sounds in his sleep for like, 10 seconds, but never wakes up. I’ve checked on him a couple times, he’s breaking fine (a tad congested, but way less so than when he’s sick), he adjusts and stays asleep, room is a good temp, and everything seems normal. I just can’t shake the anxiety of why he’s doing this. Why would he let out sad little cries so often and never wake up??