Car vibrates/grinds going down hill on highway?

Been having a vibration problem with my car at speeds above 65. Even bought new tires and had 2 mechanics look at it and said it was fine and it's either in tires or wheels. But why when i go down hill..i feel more vibration in seat and hear this low key grinding noise. Can also feel the grinding sometimes in the wheel. Now this happened when i moved front wheels to back..it made it worse. It wasnt as bad before. Why would it vibrate more going down hill and when i get on level ground..it smooths out but not always. Still vibrates randomly on smooth roads as it always has since i bought it 6 months ago..some days are really bad..other days it doesn't do it...WTF