Every 6 months or so I have to retighten my manifold bolts. The first time it happened I had no idea what it was either, sounded like a diesel VW bug until it warmed up. I posted a bunch of vids etc. Eventually someone PM'd me and told me to check those bolts. When I took off the heat shield, one bolt fell completely out and another was missing. Although I later found half of it still in there.
Just the past few weeks, it started ticking again, but it sounds a little different this time. I checked my bolts and they are all tight still. I don't see any visible cracks in any of the exhaust segments, and there no reason for the gasket to "go bad" since it is metal. It goes away very quickly after startup and isn't that noticeable in the car, so I may just live with it. If it is a cracked manifold, the options to fix it are few, hard to find, and expensive. I would probably just have it rewelded, but that's not happening until the winter.