Do you have a cat on the car?
My F150 got this LOUD knocking, you would swear the rod was about to punch out the side of the block. Closer inspection revealed it coming from the bellhousing area. All the bolts there were tight.
Then somehow I found that a bunch of the cat brick busted loose, and it was clanking around in the cat body, making more noise than I ever thought possible.
And it would only do it at idle basically. Any time I would put a load on it, the noise would go away because it would jam the particles up against what was left of the converter. But at light/no load, it would fling around causing all sorts of noise.