Everyone sleeps in cycles, form lighter sleep to deep sleep and up and down. In lighter sleep a person may stir, make noise, talk etc. Toddlers sleep cycles are different from when they are babies - so you may just being seeing a shift in the cycle and he's reacting to a dream state. As you said, new milestones do effect sleep as well.
We co-slept and my DS was hilarious as a toddler - once he demanded a cookie in his sleep.
I;m confused too - is he waking and going back to sleep or making noise in his sleep?
Just an FYI most people of any age will shift around, make noise etc while asleep.