I find my DS does this if he's overtired or not feeling well. Funnily enough, my husband does too, especially if he's too warm or dreaming. I ignore it as much as I can - I work on the principle that if I'm not sure if I need to go in, I don't need to! It really disturbed my sleep, though - I'd be woken by DS stirring and then would spend an hour trying to get back to sleep.
We splashed out and got a new baby monitor that has a button to mute all quiet sounds, and also made sure the volume was as low as it would go, with the sensor across the room from DS. It reduced the sound of the stirring and I got so much more sleep.