I have a 2004 F150 Lariat (5.4) completely stock, that has a whistle or squeal that I feel is coming from the air intake. It only does this when it's below freezing. It does it around 1500 - 2000 rpm. I did a search and the only thing I found talked about a noise from around the fog lights. This noise comes and goes as the truck shifts gears. It is very loud depending on how cold it is. Colder=louder. The dealer has heard it and has offered no solutions. It isn't there during hard acceleration, usually just during normal driving. During hwy driving at a constant speed around 65-70 mph it will settle in and whistle endlessly. If you touch the accelerator it quits. I'm so tired of it I've been considering getting a cold air kit of some sort just to try to make the noise go away. Any thoughts would be aprreciated.
Don

 

This thread may help: High Pitched Whistle Coming from Engine in Cold Weather?

In my case, the problem area was the air intake near the driver's side fenderwell (see my post #18 in that thread). Others have found the alternator was the culprit.