Howl coming from engine between 1900 and 2200 RPM's. Howl begins when outside temperature falls below 47 degrees. Does not stop no matter how long I drive if temperature below 47 degrees. Howl starts at about 1900 RPM's through about 2200 RPM's. It howls constantly at 70 MPH at this outside temperature and below. I have driven 500 miles and it doesn't stop. NEVER hear it in the summer or when outdoor temperature is above 47 degrees. I have lived with this for more than 60,000 miles. It's driving me nuts. My Ford dealer can't figure it out, neither can my neighborhood mechanic. They both want to start replacing parts without knowing what the issue is. The exterior temperature being the cause has everyone baffled.