Welcome to the site and great question. Since you say it isn't at all rpm's do you have a vacuum gauge to tell us at what rpm and how many inches of vacuum you hear this sound? I'm wondering if something like the distributor cap isn't back on "just right" and the rotor is touching..??..?? Or as JB mentions a gasket is an issue.?.? Since the manifold is different, did you verify the angles ? does the manifold actually sit "flat" to it's mating surfaces?? It's happened many times before, especially on a used manifold mounted to a different engine. Hope it helps, you have some things to look at now, Let us know what you find.