Hello, I have been having this issue with my 1990 F150 4.9L for nearly 2 years now. I have around 110k miles on the truck and don't drive it super often.

The primary issue started initially with this very loud, almost 'screaming' noise, which exclusively happened when I shifted from park into drive. After a few weeks, and a transmission fluid change, the noise now only happens when accelerating from a low speed (1st/2nd gears), along with a very slight whirring noise when idling. The screaming noise also occurs more often when I'm accelerating up a steep incline from a stop.
I've been looking around on and off trying to find an example of someone else having the same issue to no avail. Both of the mechanics I know personally that I've showed the sound to have had zero idea what it was.

I'm not too worried about it at this point as over the past 2 years the sound has not gotten any worse, but I still would like to fix it sooner than later.

The sound: http://www.youtube.com/watch?v=-ZWeq...ature=youtu.be
Also attached the same video if the link doesn't work.
The sound it makes now when accelerating is slightly different, but very similar.

This is my first time posting here and I'd seriously appreciate any help or insight I can get.
Thanks!