I have been troubleshooting this issue since I have finished a complete timing chain rebuild and oil Pump replacement on my 2005 F-150 FX4, The issue is there is a high pitched squeal and grinding or slapping sound coming from what appears to be a pulley or belt or Alternator. I have already replaced the belt and the A/C compressor Clutch and bearing and the sound continues. I have created a video for reference of the sound and the Engine, thanks.

 

Have you tried removing the belt and running it for a short time to make sure the noise isn't coming from a timing chain or something inside? That's the first step.
while the belt is off did you spin all the idler pulleys and the tensioner by hand? Make sure none feel wobbly or rough. They should spin smoothly and have no wobble.
Try that and report back

 

I have removed the belt and started the engine and the sound goes away so the issue must be with a pulley as the belt is only a couple months old with very little drive time on it. The question is which pulley could it be and or could it be the alternator making the sound as well.

 

Sometimes it can be hard to track down. Yes it could be the alternator, my last truck had a squeaky alternator.
Spin them all my hand listen for any sounds of roughness wiggle and look for any play. The tensioner pulley commonly gets wobbly and rubs on the front if the timing cover.

 

I always had good luck using just a broomstick handle to touch around the pulley , i. E on the alternator housing, Etc. And the other end to your ear. Just be very, very careful and cautious oh, that you were nowhere near the fan!! or any turning Parts with the end of the broomstick but you will definitely hear the resonance through the broomstick when you find the part