Its definitely ground speed. I have shifted gears, and the rate of rattle remains the same. The higher the speed, the higher the rattle rate. The more load "gas" I give the car, the louder the rattle, but rattle rate remains the same. Does not happen at all below 30 mph. Braking does not affect it. I cannot feel the vibration anywhere else.

This rattle is loud enough, where the radio has to be very loud to drown it out. I cannot detect any change in driveability.