What can cause a vibration during idling at red lights?

Lately, while I come to a complete stop at a red light and idle, my car starts vibrating. It is not anything violent, however, it is definitely a noticeable and fast vibration. If I put it in park (automatic transmission) it completely disappears, gone. When I put it back in to drive it comes back within 5 seconds. As soon as I accelerate from the stop the vibration completely disappears. During this whole time the RPM holds rock steady at 600 RPMs.



Does anyone have any experience with this, or has encountered a similar situation of their own?