04 Ford F150 4x4 makes a whining noise?

[ladiekiller]

04 Ford F150 4X4 makes a humming or whining sound when I let off the gas between 45-55mph. Does any one know what may be causing this Noise?

 

[Adam]

Does it ONLY happen when you let off the gas? Cause i had a loud humming noise on my ford explorer and it turned out to be the idle control valve. It would come and go at random times.