2006 Ford F-150 whistle noise heard underhood

The truck is an '06 F-150 fx4 with the 5.4L and has about 53000 km. When driving in cold weather up here I am starting to hear a very loud "whistling" or "whining" noise. The noise had never happened during idle, but instead comes and goes during cruising speeds and occasionally when the tranny is cycling through gears (always the top end of the gear). Taking my foot off the gas pedal for a moment always puts an end to the noise, but it tends to return with normal cruising rpm's. Only when the engine is completely warmed does the noise seem to disappear, and even then it returns from time to time with normal driving. So I am just wondering what could be the cause of it in this engine. I keep seeing the same three guesses at what it could be. 1) Cold air passing through the nylon tube in the idle-air bypass flow circuit or through the throttle body 2) A pulley and/or belts or altenator. 3) The fan. If anyone could shed some light on this for me so I have some ammo to take to the dealer