that is an interesting question... as i know that I get a rattle at about that speed... and i have noticed it goes away when the spoiler drops... as for vibration... not sure if that would be enough to be a culprit... I know there is a small amount of buffeting on the spoiler ( I need to adjust my hatch to stop it from rattling)

looks like in the underhood fuse panel fuse 20 is for the spoiler system. easy way to check would be to disable the spoiler, take it on a straight stretch and see if *** is still there... BTW... nice post on the tire marks BTW...

Seannon