I have the 5 speed with the v8. I believe the noise is coming from the differential in the rear. However, mine only makes the noise at around 40 to 42 mph, and at a higher pitch at around 56 to 58 mph. I also had a F-150 with a v6 with a 5 speed and it would do the same thing at around 40 to 42 and up. I'm thinking it is normal.

 

does it just do this when it is cold out?

 

no it does it everyday no matter what temp it is.

 

can you just hear the sound in your truck?

 

You know i've just started to recently hear the sound your talking about. Its like a buzz/whistle around 2000 rpm. I know the sound is comming from the fan clutch, the sound is variable with RPM. Its quite annoying. But i really dont feel like spending 100 bucks for a new clutch, or the hastle of putting it on. Temps around here are 10 degrees.