Bearing Rumble
WARNING: This page is about a different car, the 2006 Ford Pickup, 2006 Ford F550 Super Duty, 2006 Ford F450 Super Duty, and 2006 Ford Cab & Chassis. However, it is still accessible from the selected car via links, so may be relevant.
Bearing rumble sounds like marbles being tumbled. This condition is usually caused by worn/damaged wheel bearing. The lower pitch is because the wheel bearing turns at only about 1/3 of the driveshaft speed. Wheel bearing noise also may be high-pitched, similar to gear noise, but will be evident in all 4 driving modes.