I have an 2007 F150 5.4 and have noticed a slight whine when it idles. I changed the serpentine belt about a month ago (which helped, but it's still there). It doesn't get louder when I accelerate, and it doesn't sound like the transmission, but I'm totally lost on what it could be. Thoughts/ideas?

 

Does it do it with the belt off? If not, then you know it's one of the FEAD components. IF it still does it, then engine or transmission.

 

What brand serpentine belt did you use?

 

I used dayco. And I haven't tried with the belt off, I had a shop put it on since I hate changing belts.

 

Could be your Alternator recharging your battery if the noise speeds up with a higher rpm???
if so have a load test done to battery if it a few years old, may be making alternator work harder.

 

I don't think it's that, I replaced the alternator and battery a couple months ago.

 

I have found that some rebuilt alternators will have a slight whine , but work w/o any problems.

 

Tensioner? Idler pulley? I always replace those with the belt..

 

You can try this out