What causes valves on a Chevy smallblock to come loose making a tapping noise?

Mosy likely you have a camshaft that is going flat. When a piston goes up and down it pushes a rod that sits on the camshaft lobe or the high part of a cam shaft. This in turn pushes the valves. The motor spins turning the crankshaft which in turn spins the camshaft. When you have one or two valves that keep getting loose usually you have a camshaft that is going flat. This is caused by many different things. Poor installation. Not enough cam lube to keep it slipping in the valley. If one component resists even the slightest and stops during rotation this causes a lobe on the cam shaft to go flat.