I think I should hand this one off to someone who is more of a child expert, sorry.
Hi, emptyshell, I want to reassure you a bit. I rocked myself to sleep for years, and then when it was time to leave for college, I knew this would seem pretty strange and/or childish to my dorm-mates and I simply stopped. Now, it is true I had some ocd, though not related to the rocking as far as I know, but I also was very bright and very active, and I think if the rocking isn't a problem for your child (and the sound doesn't drive you nuts---if it does, perhaps use a foam mattress? or put carpet on the floor under the bedposts?), I wouldn't worry much.
Hi I don't think that it's too much to worry about. I think that they just rock themslefs to sleep as a comfort idk. My little nephew has rocked himself to sleep everysince he was able to rock and he still does it now at three. But the only thing wrong with him is that he has autisum. I dont' know if that's a factor or not. Hope this helps!