why does my baby make clicking noises while sleeping?

[PavlovtheCat]

He makes a clicking sound with his tongue/mouth like he is suckling in his sleep, does this every night for a good hour or two, sometimes more...any ideas?

My dcs both do this, they are now 4yrs and 3yrs! Think it's just a comfort thing stemming from when I used to breast feed them, which I only did for 11 and 8 months respectively. Dd does it as she's smelling her ted's paw, I think it's cute!