My baby too! She's 6 wks old now, and we call her Grunty McGee sometimes, and laugh at all the grunts and groans she constantly comes out with. We even had to move her moses basket out of our room fairly early, since she was so loud while sleeping that i'd be up most of the night listening to her, wondering when she was going to wake up and actually be hungry.
I don't think it's anything to worry about at all, though - we just laugh at it and tease her for the unladylike noises she comes out with. It's somehow fitting since our main nickname for her is Willa Gorilla. It definitely doesn't seem to be associated with pooping or discomfort of any kind. I'm sure she (and your ds) will grow out of it.... Good luck with the sleeping, though! I told my husband recently that I'm not sure what we would have done if we had decided to have a family bed, since there's no way we (and especially me) would have gotten any sleep.