My Utu does it a lot actually. Almost daily. It's 'gotten worse', meaning that he started when he was around 2 years old and used to do it randomly maybe once or twice a month and now he is 4 and is doing it more often.
I remember the first time he did it, the other cats (and I) were like "what the...?!". He sounded totally different than when awake. He sounded almost like a kitten. Now he's gotten louder, sounds more like himself, and usually even wakes up to his own meowing. The other kitties have gotten used to it but I still find it odd/funny/confusing. He doesn't seem to mind, just goes back to sleep.
He does lots of things while sleeping, paws, whiskers, tail and mouth going all the time, much more than my other cats. He usually sleeps belly up, looking like a passed out person or a road kill. My ex's friend used to call him 'a cheap woman' because of this.