What’s the best way to create animated ‘heat waves’ that you would see coming off the top of a hot car hood, or behind the afterburner on a jet?
I was thinking about possibly using transparent metaballs with a little fresnel. What have you guys used?
I would use particles with a transparent material and some IOR to distort the background.
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Serialsiner
Excellent. 
I am visualizing this as some kind of an effect that you would achieve using a Sequence Editor filter. I’m not sure what one it would be, or if maybe it would have to be a custom one. But basically, a layer consisting of a map, generated (say) with a particle system, would provide input for distorting the underlying portion of the image. The darker the map-area, the more input would be taken from “nearby” pixels. But the bottom line is that this is, to me, an effect that is most easily described in two dimensions… and applied to the graphic after it has been rendered.
I have come to realize that heat distortion is commonly implemented during the compositing stage or as a post process. However, this doesn’t give the amount of flexibility that after.
i would make several ‘wiggely’ object placing them above the hood. then adding a slight cloud texture or maybe a stucci to accevy the distotedness. then finnally make it transparent to the point that you can barely see the distortedness of the texture