It could very well have been a leak in the intake somewhere else too. Also consider that most "cold air" intakes like the k&n are actually pulling hotter air into the engine (several people have confirmed by measuring intake air temps with a scan tool). Search as there are plenty of threads about cleaning the IAC valve. Personally, in both cases (97 4.0L and 98 4.0L) I tried to clean the IAC valve on my trucks, it made the noise louder. The noise they produce is usually a fog horn type of noise, I wouldn't really consider it a high-pitched whine, and generally only happens at idle.
2006 F150 FX4 Super Crew - Bilstien 5100s leveled 2", 35" KM2s, 4.56 gears, Detroit locker, Eaton E-Locker, Traction Bars, AEM intake, Xcal 2
2010 VW Golf TDI 6MT