I had a baby that made noise like that.  She was never in pain.  She grew
out of it but I think it was because she was looking for her Mom.  If I
went to hold her, she would stop.  She would climb up and get into my hair
(around my shoulders) and fall asleep.  I still spoil that little ..well
not so little any more..one (and my 6 others!)
[Posted in FML issue 3519]