What would cause a loud tapping noise under the valve cover that gets?

if you think about whats underneath the tappet cover, it should be noises that's made by the tappets or the mechanisms that drives the valves.
if the valves are driven by an over head camshaft, there could be play between the camshaft and the rockers that pushes the valves down, or play between the rockers and the lash pads at the top of the valve. This can usually be adjusted by a mechanic.
it your car is push rod driven, you also need to check for play on your valve train.
other things that can make a tapping noise could be a intentioned timing chain that hits against the walls of the head or the block.