My X only has 500 miles on it so far, today I noticed a slight vibration/shiver while gently holding down the gas doing 40-45 mph, but it only does it when the tranny is in 5th gear at aprox 1400-1600 rpm's! I think it may be normal because the tranny is kicking down to O/D when the vehicle really just doesen't have enough speed for O/D yet! Does that make sense? any input?