WHY DOES LINCOLN MKX MAKES ROARING SOUND BETWEEN 40-75 MPH

Things to check (in order of likelihood):

1. Fluid level and condition in rear differential;
2. Drive shaft center bearing;
3. Rear outer bearings;
4. Front bearings (have they been packed with grease in the past three years?);
5. U-Joints
6. Tires (separating tread, flat spots, etc.)