If I'm not mistaken, I belive it is the same as us proteges, if it is, it is th intake manifold runner. just buterfly valves in the intake mani that stay closed during low rpm and open up in higher rpm. Nothing to really worry about. just hea to your local Auto Zone and have in cleared for free. It may pop up again.
But just make sure all of the vacum lines are hooked up and not cracked. there is a cylinder on one side of the intake (passenger side on the Pro's) with a couple vac lines comeing off of it, it is somewhat difficuly to get to and see. but that is what controls the runners to open and close them.
