Ideas on what's causing my click/tick/rattle noise?

Not just any exhaust leak, a leak between the head and the headers. Was the mating surface on the headers flat? did you use some nice thick gaskets? Think about this scenario - The exhaust stroke puts pressure against the pipe and the exhaust bolts and pushes the header away from the head. Closed valve and it snaps back in place and anywhere the header flange hits the head causes a tick. The higher the RPMs the worse it sounds.First try re-torquing the exhaust bolts to see if it changes at all. Try tightening the bolts with engine running and ticking if you can get to them without burning the $&#! out of yourself. I've had this problem on small blocks.