I know you mentioned you were going to, but did you end up checking the Crank sensor at the back of the block? Had many FWD's do that after putting them back in. They usually sound different than that but It would suck to pull it all apart for something on the outside. I wouldn't trust just looking at the flexplate for evidence, see if you can move it slightly away from the flexplate. The noise sounds way to constant to be a vlave guide or pushrod at that rpm.
Just ideas.