I really can't think of any other way to describe it! I assume he's straining with wind, although nothing seems to come out.
He wakes any time from 2:30am-5:30am after his 10:30pm feed, and for the past 2-3 weeks he's been waking with a really strained moaning grunting noise. imagine doing labour mooing, with your mouth closed, and that's what it sounds like. he also makes this noise when he wakes in the morning (7-7:45am), but not after any of his other sleeps.
I'm wondering if it's linked to his useless gut (see my thread here), but whatever it is it's a hideous noise, and I really feel that if he wasn't waking up uncomfortable, he'd be going a lot longer at night - on nights where he wakes up just whinging for a feed, he tends not to wake till 5:30, or even sleep through!
He's 18 weeks, btw.