It's probably the inner and outer portions of the trunk lid vibrating together. Drop the on of the rear seat backs and hit the highway... see if you can locate it that way.

 

Are your windows up or down at that speed? I could see wind buffeting (major pressure and low frequency vibration) sounding how you describe at those speeds.

Don

 

Put the rear seats down at speed and see if it changes the sound. Also I had in my 06 on the back deck lid these 2 plastic covers that would sometimes vibrate. Very annoying. Put some tape around the area and it went away.