I have a 2005 Ford F150 that has a high pitched whistle at about 1800-2000 RPMs when it is extremely cold (20 degrees or colder). The whistle goes away if you speed up or take your foot off of the accelerator. I have looked tiressly on the internet and have found that many Ford F150 owners have the same problem with no solutions. It can’t be a vacuum leak if so many have the same problem occuring at the same RPMs and temps. It has to be a common imperfection in design. Does anyone have an answer?
I would look at the air induction system. All components from the air filter box to the throttle body.
I also found this TSB http://www.ford-trucks.com/tsb/fulltext/show_article.php?tsb=02-10-8#fig1
Tester