Ok so basically i have this menu and on rollover i want the green background to slide in from the right and BOUNCE back to the place shown below.

Is there a javascript for that?

So basically the bar appears and slides from "under" the white layer which is shown on the left to the left underneath the menu text and keeps moving and than like an elastic band pops to a predefined point (shown in the image)

Hope someone can give me a little nudge to the right way.

Click to view attachment

Not 100% sure what you're asking, but i can give two hints:

1. Use flash. Flash is the most common solution for animations, and so would probably be the simplest solution.
2. Attempt to use JS to adjust the background-position of the navigation. You can use setTimeout() to make the animation relatively smooth. Be warned though, this sort of solution could get quite complex.