Intermittent Vibration from Rear of Car
 

Cruising down the highway it will suddenly start vibrating. Not a severe vibration, but engough that you can feel it and enought to annoy you. It's only when I'm coasting or light on the gas. If I hit a bump it goes away. It's like something hits a harmonic and starts resonating. It doesn't do it around town or when I'm getting on it. Anyone have any ideas?

-Lars