So I thought this was due to D teething and feeling discomfort when lying down but it seems not....
D is 20mths old and for about 3mths now he does this 'thing' when we put him down to nap or sleep. Usual routine, lay him down and then he goes 'aaah, aaaah!'. Sometimes before I've even left the room, sometimes it can be up to a min or 2 before he starts. He makes the noise, I wait and see if he'll stop and settle, he doesn't. I go in, put my hand on his bum, go 'shhhh' and leave again straight away, no messing. Just to add when he calls out and I go in he's not sat up or anything, he's just lying there, doesn't move in response to my coming in to reassure him.
Sometimes I only have to go in a couple of times, other nights I'm in and out like a yoyo. some nights i'm back in as soon as I go out, other nights the calling out is several minutes apart. So no rhyme or reason to it. he also wakes usually once a night to do the same. When he does this I just reassure as before and sit in the chair in his room. in general he's back asleep in minutes. it's never the same time that he wakes each night and every few nights (maybe once a week or so) he sleeps through.
I know this is no big deal compared to what others are going through with sleeping but I just don't know why he's doing it or what to do about it! All I can think is it's a habit from when he was teething his canines.