My '02 F-150 with a 4.6 whistles when it's cold out. The whistling only occurs between 1300-1400 RPM, and only when the outside temp is cold, around here that means below 40 degrees F. It doesn't go away when the motor gets hot and it happens at the same rpm if I'm in park or in drive. Unfortunately I seem to spend a lot of time at this RPM so this is really annoying.

It's always done this since I bought it new in late '01 but it seems to be a bit louder this year, or maybe I'm just less tolerant of noise than I use to be.

The truck is bone stock with the original intake, air filter, motor, trans, etc.

I've got a few days off of work coming up and I'm going to try and track this down. I'm looking for any ideas or suggestions about what this could be and what to look for.

 

check tranny fluid level and all ujoints and diff fluids.

 

I wouldn't doubt it if the sum beech isn't coming from the tensioner pulley and/or the idler pulley. I have the same problem and it actually will get worse where it happens no matter the temp. Of course now that I have narrowed it down to those mine does not do it anymore. Gotta love these Fords, they fix themselves, on more occasions than once. Mine actually makes or made the noise @ 2000 rpm steady, but from 1600-2200.