This is too funny. My 2004 1/2 also does it. I've had it since new and all these years I've lived with it come winter. It's more of a drone whistle and only happens when it's below 12°F or so and it only happens as a certain engine loading. One trip back fron upstate NY it was -25° and it was pretty constant. I allways thought it was the air intake and I've just gotten used to it.

 

I have the dreaded squeal on my 98 Expy with 122k miles. It is getting worse with time's passing. So I figure some mechanical component will give up the ghost before too long. The squeal occurs when cold (30F-40F), as others have experienced. The pitch changes with RPM, but seems worse when the engine is not under load. I was thinking alternator, power steering pump, fan clutch, idler, or water pump. If I get it fixed, I'll post the result. It is consoling to know that I do not suffer alone.

Update: I changed out the air filter, today. My old filter was a K&N unit that had become quite dirty. The foam gaskets that seal the filter to the housing had come loose from the filter body in a couple spots. The adhesive tape that K&N uses to attach the foam gaskets to the plastic housing is not very durable. This is the second K&N filter that has had this problem. Anyway, after I installed the new pleated paper air filter, any trace of squeal was gone. It seems hard to believe that whistling air was causing the noise, but that seems to be the case. I'll have to wait until it is cold again to confirm the problem is gone. I took the truck out twice today after the air filter change, and in both cases, there was no hint of the squealing noise that I had yesterday afternoon. Maybe there is no correlation to the cold temperature, and it was just dumb luck that my filter started whistling about the time it was getting cold. If anything changes over the next few days, I'll update my post.

Well, it has been a week with no problems and no noises. The old beast runs like a new one. I did clean the mass airflow sensor. Apparently it was contaminated with K&N oil. Since cleaning the MAF, my engine trouble codes have not returned. K&N has a new vastly improved gasket they'll send you if you have the older version of this filter. I got mine for free by requesting warranty service on my old filter. It appears this new gasket will last as long as the filter. The gasket wraps around the filter body, and appears to be made of a rubber-like material, instead of foam.

 

Thanks for the information. That was one of the things I had on the list of things it could be. I have a fairly new air filter installed, but I've noticed the entire housing is really lose so it's possible the noise might be coming from there. Thanks again for the help and keep us informed!

 

I had a high pitched squeal coming from my 08 f150. This was the problem. https://www.ford-trucks.com/forums/8...b-finally.html

This may help some of you, but it does specify an 07-08. Still, you never know.