Exhaust manifolds are pretty rugged. Maybe the gasket? How about some of the shielding? Most of my shielding has rusted off, but rattled like mad when it first came loose.
Have you (do you need to) adjust your valves? I forget how often it is called for on my '90 8v (maybe every 20k), but I know my valves quieted down quite a bit after the last adjustment.