Some of our cats will moan, or chitter if they're dreaming, and that's quite normal. But not so with Efghenye. He has asthma, and is on an inhalant. X-rays revealed scarred lungs from the asthma. At present, one of our new rescues, Purna, is quite noisy when she sleeps, and the vet hears raspiness in her lungs. We've been told to keep her on a course of Panacur in case she has roundworms or lungworms in her lungs, and if that doesn't work, we're to have her x-rayed for possible asthma or other problems. Sometimes heart problems cause an odd sound when the cat breaths.
But your cat's probably fine.