I have also read on this board that if you can't hear under the hood but you can hear it in the cab. Turn off your radio and see if it goes away. I know they (Ford) has had a few issues with radio noise suppression.

You might also just have a plain old vacuum leak. Check the famous elbo tube right up next to the fire wall.

 

My clutch was real loud and could hear when I started the engine so I knew when it wasn't working right. When I got the new clutches in the box it explained how to check it. It had some procedure to follow like warming up the engine then shutting it down and trying to manually turn the fan by hand. It's supposed to be locked up. My original clutch that was making noise appeared fine when I tried to check it with their procedure. Their procedure checks the fluid inside and mine was OK there but it was the bearings that were shot on mine. With the belt on it I couldn't feel the bearings being bad because I couldn't spin it fast enough under the hood. Once I took the clutch off I could freewheel spin it by hand then could hear and feel the bad bearings inside. I think the general rule is that if fluid is leaking out it's shot. Mine happened to be the bearings. Next time you change the belt give all of the pulleys a spin while the belt is off. You'll be able to feel the bad bearings.