ARrado15

So I've noticed that when i'm on the highway and started going over 70 mph that there would be vibrations in the car. Kind of like a wheel is out of balance, but then I noticed that i get the same vibration when in 4th, 5th, or 6th gear right at about 3k rpms. It doesn't do it in the lower gears, and I noticed when in 6th that it is worse when accelerating, even if a very tiny amount. I don't really know how to explain it but it kind of feels like when your in a high gear and going slow and about to stall... would you guys know what i should check