C.R.
No need for alarm. My son did the very same thing. According to "What to expect:the first year" the head shaking is his way of learning that he has rhythm. My son would do it so violently that it would move his walker. We just played like he was saying "no,no,no." I even had video of it to confirm his motions with my ped. Be forewarned, my son now rocks as well. His father did it, and so did his father. All perfectly normal.