Whistling Noise
 

I have a '03 7.3L that makes a high/low pitch whistling noise when driving between 10mph and 45mph. I find it similar to a bobwhite quail whistle. It might make the noise at higher speeds, but I probably can't hear it because of road noise. It started last winter, went away during the summer, and is now back again this winter. It seems to only make the noise when driving. If parked and the engine is revved, I don't hear it. Any ideas as to the problem??