Is it a snoring or does it sound like he's actually having trouble getting air?Of course you know it could be 2 things: Just dreaming, or actual issue. Just keep an eye on him and see what happens.
Ziggy snorts in his dreams, especially if the dream requires him to run.
I guess he gets out of breath in his dream too, that's when he makes the weird little noise and a few seconds later a sigh or more dramatically, a huff. The little stinker is almost as entertaining sleeping as he is awake.