18 inch Tires ever so slightly out of Balance
We rarely drove on the highway for the first month or so of ownership and the vibration was so subtle that it was hard to be sure that there even was a problem. When we did take the car over 50mph for a more extended period we realized that indeed it was a slight vibration, like a subtle low energy hum, a washer with an ever so slightly out of balance spin cycle. I thought lumpy tires that had been sitting on the dealer floor for months without moving might be the cause. Then I asked myself what if I had to spend a whole day driving like that, no I'm taking it to the dealer. I took it for a ride with a tech in the car and he said, "oh yeah, I feel it, probably out of balance tires". I did notice that one of the wheels had an absolute ton of balance weights on one side and the tech said he took that tire off the rim spun it 180° on the rim and rebalanced. Viola, problem gone.