Hard to know. I'd suggest you use your phone (or someone else's) to record the audio and even the video at the same time, and show it to the pediatrician. That will be far more helpful than trying to describe it. I would do that with any baby, but especially a preemie. You'll feel better if you do, and if there's any reason for concern, the sooner you alert the doctor, the better.
Updated
Hard to know. I'd suggest you use your phone (or someone else's) to record the audio and even the video at the same time, and show it to the pediatrician. That will be far more helpful than trying to describe it. I would do that with any baby, but especially a preemie. You'll feel better if you do, and if there's any reason for concern, the sooner you alert the doctor, the better.