Go underneath and check the pulley to harmonic balancer bolts. You may have one or a couple bolts too long making them scrape against the timing case cover.
Had this happen on a 68 Roadrunner with 383. We did a new rebuild. On start up we get this noise we could not find.
With the engine off, went underneath , noticed that the pulley to harmonic balancer bolts were too long and scraping the cover. We put shorter bolts on and no more noise.