I noticed that the vibration was associated with rpm - so if I was in a lower gear but rpm was the same as that at the 68-72 mph then I had the vibrationSpeaking of throwing rocks..... and I hate to bring this up.... but..... I have noticed that there is also a vibration at 38 or 39 mph that is similar to the vibration at 68 or 69 mph - just less vibration due to less speed. I am hoping that if they fix the 68 mph vibration it will fix this one too.