Originally Posted by mccorry

Mine does it in a straight line... not while turning, Blue.... at 40MPH at 5000 RPM. The tires aren't spinning.

IMHO, It is a harmoic vibration that is coming through the floorboard... either a bushing that is less pliable at colder temperatures... or a rub. I am not sure which. My money is on stiff rubber bushings. The vibration of the drivetrain is getting passed right into the cabin due the the bushings being cold and stiff.