It most likely isn't anything to worry about. Some cats make noise when they sleep. There was a thread here about cats snoring not too long ago. When I first got my cats I noticed my boy-cat Rocket made noise when he sleeps, as well as twitching movements. These things in humans can lead to sleep disruption and sleep deprivation, so naturally I was worried about it. But it turns out that lots of cats do that.
If his breathing is clear and unobstructed while he's awake, it's probably not a breathing problem, such as asthma, which might be the only concern.
As long as he gets restorative sleep, which you can determine by his behavior during wake time: does he have normal energy level, does he engage in play, have good appetite, normal behavior, etc.
If it troubles you, maybe you should contact a vet about it. Of course, you won't be able to duplicate it in an office visit, so your time and money might be wasted. I don't think they do sleep studies on pets!
But at least you would be reassured.