I have an 05 with a 5.4 and it just started doing this couple nights ago while it was -20 degrees. Looks like many here are experiencing different whistling problems. This whistling is not in the alternator, as it does not do it if the pickup is parked. Only while going down the highway at 71 mph with rpm of 1950. Let off the gas just a little, it goes away, get on the gas just a little, it goes away. It has also done it at a couple of lower speeds, but not as distinct as when it is at 71 mph. It does not go away once pickup is warmed or driven for a long period. It is also parked in heated garage, and the noise starts right away.

Are the tach, and speedo all electronic, or are there some mechanical parts to them that would react to the cold weather?