Very loud engine knocking sound at low RPM's (Attached Video)
 

2008 5.4 L 4x4 F150
171,000 Miles
VIDEO LINK AT BOTTOM

This problem began about 4 weeks ago now and has only gotten worse.
Problem: Extremely loud and obnoxious engine knocking sound, almost sounds like the engine is seizing up
When Does it occur: Only after the truck has warmed up for about 10 or 15 minutes. And only when at very low RPM's, most noticeable when slowing to a stop at a light, backing into a parking space, or slowly merging in slow traffic.
What have i done to diagnose: Im no car expert, the most work ive done is change a starter and some oxygen sensors. So i dont know where to start when looking for whats causing the problem. But i did check the engine and trans oil levels and they are both fine.

I have attached a video of the problem so you can hear the noise. Thank you in advance

VIDEO

Check out this video by Ford Tech Makuloco. Sounds almost exactly like what you've got in the video.