so i havent touched my truck since i bought it, but it has been acting up a bit.
a couple cold mornings it would whistle when i fired it up and drove it with out letting it warm up. the whistle tone was direclty corelated to rpm. and would last for at least 10-20 min coming and going.
recently the problem has become more frequent. i took a 4 hour round trip yesterday and was annoyed but able to pinpoint the noise a little.
it whistles from 55-65 mph only when the rpms are between 1750 and 2000. essentially in 4th gear only and only about 80% of the time.
at 1800rpm at 30 mph it purrs like a kitten. if its underload like going up a hill at 65 at 3000 rpm it is quiet. at its loudest the pitch can be changed with an rpm change so im thinking intake ish but im not sure, anyone have a clue?

 

Check your exhaust manifolds for leaks as well. Some of the guys on here have had alternator drive end bearings whistle also. If you have your alternator rebuilt request German or Swiss made bearings, most of them use Chinese bearings

 

I have an 04 f150 5.4l, mine does the same thing and i opened the hood, had my brother come out to listen for it, and i gave it some gas while it was in neutral and it sounded like it was coming from the alternator. Thats where im at for figuring out what the problem is. if you find out better info feel free to let me know!

 

yours is probably alternator bearings. ive read alot of other people have had chinese bearings whistle, when you go to replace have it rebuild with quality bearings. most let it ride and ignore it until the alt dies. in my case i think its defferent because it is only at speeds greater than 55, it wont do it slower of at idle unless its really cold.

 

yea thats what i figured. mine whistles randomly at anytime. somtimes its just a constant whistle getting louder while accelerating and a low whistle at idle. for some reason when i have my radio on and my auxiliary plugged in, it amplifies the sound. my programmer shows that im getting good voltage so the alternator isn't failing yet. lets just see how long it takes before i change it out from getting annoyed.