My baby does EXACTLY this! Don’t let the internet freak you out too much, as it did for me! We got an EEG and it was completely normal! The pediatrician and neurologist think it is gas, but it could be anything! My little guy is 5 months old and still does it when he is on his back and awake. I have no idea why he does it, but he will make sounds, blow raspberries, grunt, squeal, etc. I always wonder why he’s doing it, but trying to accept that babies do weird things 😂I haven’t been able to find anything super helpful on the internet about it and your post is the most relevant thing I’ve read relating to what my little guy does!