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Hi! Congratulations on your new baby! I'm happy to hear he's finally home!
It's not unusual for babies to make grunting noises (I always tell my patients' parents that they just do it to freak them out :o)
Here's what to watch for: If he's breathing more than 60 times per minute consistently (it's not uncommon for newborns to have short periods of rapid breathing that cycles with periods of slower breathing and that's completely normal) AND/OR if he seems distressed with his breathing (for example, you see his belly really moving in and out when he's breathing or if you can see the skin retract between his ribs when he's breathing)
If he's feeding well for you, his temperature is normal, and he's not consistently having the grunting or the signs I mentioned above it sounds like normal baby behavior.
Please let me know if I haven't completely answered your question.
It can be completely nerve wracking! It's certainly possible that his days and nights are a bit mixed up. It's pretty much always day in the NICU, so it may take a little time for him to work the schedule out. The grunting when he moves around sounds completely like normal baby stuff and I would not be concerned given everything else you've told me.