Baby moaning/whining all night but not waking up?

Our 11 month old has been whining in his sleep most nights for the last two months. Some nights just a little bit; other nights, hours on end to the point I have to believe his sleep is affected. I will lie awake at 2 or 3 AM just hearing a steady stream of moans and whines, watching him wiggle and toss a bit but never actually wake. While he may punctuate with a cry out for a second or two here or there, he never actually starts crying.

Rarely does he actually wake up at night. He has a twin that doesn't seem bothered by it, and we found that waking him up to check his diaper, offer a bottle, check temp, cuddle, etc often turned into an all night ordeal to get him to go back to sleep (he wants to play).

His energy and appetite levels are normal during the day and he naps ok (except for occasional nap whining). He falls right asleep at bedtime at 7 pm and usually doesn't actually wake until around 6 or 630 AM.

We've thought it could be the fact he and his brother have had like 4 colds in two months and he might not feel good, or the fact that the heater turns on and dries out the room a bit (we do run a humidifier) or the fact that he may not be getting enough solids during the day (we offer them 3 meals plus snacks and let them eat until they start playing). He's a big guy with a healthy appetite so I would hate for him to be hungry or thirsty at night. He could be teething; he has 6 so far but I haven't seen anything cutting through in quite some time and 2 months of the night whines seems extreme. All of these feel like grasping at straws.

The only real pattern I could detect was that he had started the whining the night after we had dropped from 5 bottles to 4, and moved the bottles from before naps to after. This doesn't make sense though as a cause; he still gets dinner plus a bedtime bottle, so I don't think he's hungry. Plus, his peds wants him and his brother down to the equivalent of 3 bottles by 12 months (in 3 weeks, I have no idea how we will accomplish that but that's a thread for another day).

Anyway - is this any reason to be concerned? Should we be waking him up to check on him? Any one else have a night whiner?