Pulsation in left upper back

I'm a healthy 22 year old female, over the past few months, I've noticed that when I lay down, I can feel my heart beating on the left side of my spine. Also I've noticed that when I sit with my legs crossed, my body tends to rock slightly insync with my heartbeat. Should I be worried about my aorta?

[Jerry_NJ]

I take it from your brief post that you do not have any other symptoms, nor have you been examined by a doctor relative to your current symptoms.  I also take it the heart beat is regular and about 1 per second at rest, i.e., very normal.  If not tell us more.

In the absence of any other symptoms I think it good to try to put the subject our of your mind, and perhaps trying resting in a different position, e.g., on your side, or other side, or back... and see it the pulsation diminishes.  

[stutterheart1]

You are probably just feeling your pulse in places you are not used to. When I turned about 19, I started being aware of my heart beat when I layed down and could feel it in my neck when I walked up stairs.
Your symptoms aren't indicitave of anything abnormal.