Originally posted by Miles81_240d
........I keep wondering if there is some sort of "slack" in the driveline somewhere... possibly in the differential (?)
Exactly my point. I can usually spot a "cycle" when I see one, but that's not what this is.
Picture sitting still in the drivers seat of your car, and some one is behind the car pushing and pulling it forward and backward in quick succession. Literally no more than a second or two between pushes. And they're pushing it hard enough to jerk your head (definitely no pun intended) back and forth.
That's what this feels like. Only it does it at 70 mph!