Rocking on all fours, and humming as he rocks. He does it for long stretches of time, several times a day. The first time I noticed, he spent the better part of the morning doing this. The strange thing is that the kid can crawl - he's been crawling for months and easily navigates the entire house, but this is almost like pre-crawling behavior.
He does it in his crib, too, before he falls asleep, and upon waking. Last night in the middle of the night, Michael and I were awakened by a rhythmic knocking sound that seemed to be coming from Finn's room (which shares a wall with our room). Michael got up to check on him and found him up on all fours, rocking, in his crib, in his sleep! The knocking sound was his head knocking against the changing table attached to his crib.
Not sure what to make of this. I mean, it's not hurting anything . . . but weird.