Animating circles with D3.js

So I have been messing around with D3.js for a couple of days now and I have basic circle generation / animation (that act as bubbles), and I was wondering how I could animate the circles on the x axsis, so they wobble back forth as the travel / transition to the top of the page. The current animation can be viewed at chrisrjones.com/bubbles-v1.html