moejoe

I've seen a lot of comments on driveline and steering wheel vibration here, but Im feeling something that I believe is different. I just took two long road trips in my new 6MT HPT I experienced this at speeds around 80 on certain stretches of highway. The feeling is the whole car slightly shimmying or shaking side to side, but I feel nothing through the steering wheel. My truck will rock forward and back on certain roads but this is very wierd and unsettling. It has happened three times in my first 1500 miles and when I slowed down it went away. Other than this, there are no other vibration or steering issues at any speed. My impression is that a wheel out of balance will be felt on any surface as various speeds. Anyone felt this? Simple harmonics at play or something else? Thoughts?