I bet it is a normal sound. My kid makes all kinds of noises while sleeping. So do you, so do I, so does everyone. You are probably focusing on it because you have no other children to compare it to. We have ALL been there with our first children. I am sure your doctor hears about funny noises and movements and everything else under the sun, so if he says don't worry....Plus kids are NOTORIOUSLY restless sleepers, I am surprised this is the first thing to stick out while baby is sleeping.
My kid used to twitch and just like spaz out and I freaked OUT! My doctor came back to me with, babies and kids are literally "little" people. They really do go through all the same stuff physiologically we do. Sometimes they won't eat, sometimes they won't sleep. They makes noises, have mood swings really just everything we do. If you think like that, a lot of what you panic over will seem more normal.
As long as baby doesn't hold his breath for longer than like 20 seconds, then I wouldn't worry. Fast breathing periodically while sleeping is normal too. Kinda sounds like they are panting with a little vocalizations. Like dogs do lol.
Try not to worry. I bet baby is fine. Let us know though.