I have a 2003 MPV that has start making a humming sound at speeds over 30 miles per hour. It seems to be fine at the lower speeds,but when I drive over 40 mph it starts making a noticible humming howling nosie. And at higher speeds it almost sounds like is not changing gears,but when I press on the gas it picks up speed and the rpm gauge seems to indicate gears shifting. Does anyone know what this could be.