Judging from my truck, I'm sure its the air intake! It sucks up air from a hole on the driverside and at 1800-2000 rpm its makes like a small whistle sound. Mine did it stock and now with the aftermaket intake its even louder! And like you said at 1800 rpm its steady whistling high pitch sound; i'm 90% thats what your hearing! I'll film it with and without the aftermarket intake (stock is just a little bit more quiet)and you'll hear it for yourself but all I know is that its not harmful for nothing! I know for sure its the intake or maybe throttle body. Don't worry and don't call it a problem lol!!!
Last edited by 07F150OAKLEY; 02-08-2009 at 03:53 AM.