Here's the thing:
MANY babies and kids, intermittently "wake" during a nap.
BUT... this does not mean... "you" have to then, wake them up just to rock them or etc.
What it means is: the baby/child, like an adult... goes through sporadic sleep rhythms/stages of sleep/deep sleep etc. AND yes, intermittently a baby may 'seem' to wake or make noises or etc.
BUT... if 'you' then wake the baby/child... they will then, actually WAKE... and not be able to go back to sleep.
So, the thing is: know your baby's cues... and the difference between him WAKING up and crying/screaming/needing a feeding or you... OR, if he is just having a normal sleep and is just making noises or moving a bit in the crib. I mean, no one sleeps still as a statue. Adults, babies, children all toss and turn when they sleep, and/or make noises.
MY kids as babies, would OFTEN make noises and fidget around in bed... BUT they were, just fine. They were, not fully awake but just making noises in their sleep etc., and they then on their own, went back to sleep. But, they only did that, IF "I" did not wake them. So, I let them.... go about their own sleep patterns.
BUT, I also fully knew, my kids cues and noises and which noises or movements... meant, that they were just tossing in their sleep or if they needed me and a feeding.
Sometimes, it is the parent... that is 'waking' the baby. Because, their cues and patterns of sleep is not understood.
Not all noises/tossing and turning in a baby... means that you have to actually wake them up or pick them up or talk to them.
It is often, that the baby is just making normal.... noises as they sleep and/or tossing around.
15 months is also a growth-period, and they go through a lot of changes.